The problems of dealing with migrants, refugees and settlers are an area that empires have had a decisive advantage in, historically. States founded on civic equality, constitutional rights and unitary territorial integrity do not have the mechanisms of personal control, the corpus domini (body of the lord) as opposed to corpus civitatis (civic body) style of governing in a patrimonial and seignorial fashion. As such, they suffer terribly when dealing with exceptional disasters like refugee crises.
Understandably, the present right-wing zeitgeist basically involves an instinctive gag reflex at the mention of the word "migrant," with the connotations of criminal non-white hordes that it evokes. We don't want migrants. End of story.
Europe has had plenty of its own internal migration crises throughout its own history, however. In addition, the question of how to govern foreign and settler populations is intimately linked to the process of how states emerge from rudimentary associations of clans. "Race is like an extended family," people like to say now. As if it's a given that you can just seamlessly interact with people from your uncle right up to your sixteenth cousin like it's a smooth continuum of tit-for-tat kin altruism down to the bottom of the tree. Also as if it's a given that I enjoy being with my mother-in-law or something.
Now unless you intend on practicing Joseon isolationism, you are going to expand and conquer new lands, coming as they do with new people. The modern reaction to this involves throwing a fit over how the RIGHT TO NATIONAL SELF-DETERMINATION OF PEOPLES is being violated.
Actually, there's a particularly aprioristic and even libertarian way of handling this question that is popular. It involves simply assuming the problem away. "We will have a world of zero immigration ethnostates where refugees and migrants don't exist by definition." No you won't. Well, maybe. But not any sooner than we get Libertopia with its private nukes and Hoppean residential covenants.
Gens did not have to be purely blood-based, they could also be adoptive. The most notable example was probably when the Polish szlachta adopted the Lithuanian boyars into their heraldic clans after the Union of Horodlo in 1413, in a process called adopcja herbowa. The "nation" by this older definition was a practical synonym for the First and Second Estates (in the Gallic sense) represented in a corporate assembly.
(Similar phenomena were the immatriculation of Russian families into the Swedish nobility during the seventeenth century, and on the other side the integration of Baltic Germans from Livonia in the eighteenth century into the Russian nobility after the Great Northern War. The famous White general Pyotr Wrangel was a descendant of such a family.)
The Schmalkaldic League of Lutheran princes in the Reich during the 1530s and 1540s produced various exiles after the failed war in Saxony of 1546-1547, most notably the reformer Martin Bucer, who went to England and became a significant influence in the revision of the Book of Common Prayer, thus leaving a decisive legacy regarding the direction of Anglicanism.
How could have Mazzini "liberated" Italy without the help offered by his exile in London? With Thomas Carlyle's gracious defense, too. England would also be the home of about 125 Czech radicals fleeing Saxony and Baden after 1849. Louis Blanc, Ledru-Rollin and Félix Pyat were a few of many French revolutionaries disembarking as well.
Had there been no German Forty-Eighters in America, the abolitionist movement would have probably looked very different. No Carl Schurz, either. Wheeling, Virginia was especially a hotbed of radicalism, with calls for world revolution. In 1852, Charles Goepp declared that "A war to extend our [America's] institutions is not a war of conquest; for, in as much as the spirit of our government is the principle of self-government, or rather of non-government, its expansion does not necessitate the introduction of violence, but rather the abolition of the latter." Max Boot would be proud.
Plenty of Orthodox Serbs were fleeing into the "military krajina" during the Ottoman-Habsburg wars. The "Vlach statutes" (Statuta Valachorum) are 17th-century documents indicating how these refugees were to be administered. They were severed from the jurisdiction of the Croatian Parliament and made wards of Vienna, particularly the War Council. The Serbs were bound to a classic military-tenurial model, not unlike a knight-service, but more individualistic. They could elect local officials. These Grenzers would prove valuable in border skirmishing. What helped was that the old dynastic idea of a "march land" survived, allowing the border to be treated as an autonomous military zone in its own right, rather than the incoherent modern Westphalian concept of "borders" simply being the outermost boundaries of a territory, not special in any particular way except the method of law enforcement. If any.
The cause of naturalizing foreign-born Protestants to invoke an image of a safe haven against continental despotism was one that had gained traction in England centuries ago. The plan for a colony to house the refugees from the Genevan Revolution of 1782 is a good example, but an earlier and vigorous statement of this is from Josiah Tucker's pamphlet Reflections on the Expediency of a Law for the Naturalization of Foreign Protestants (1752).
Dean Tucker's main argument is one that is still heard a lot today: immigrants are needed to augment a dwindling domestic population. The case was likely especially forceful given that classical and humanist ideas about the value of large populations were probably then in vogue.
Dean Tucker also very interestingly flipped the picture on who benefits from migration as the anti-immigrant position is usually stated nowadays. The usual cliche is that immigration is encouraged by global capitalist interests to the detriment of everyone else. Tucker, on the other hand, portrayed the merchant classes of London as the self-centered anti-immigrant lobby, with king and parliament as wise elucidators of the common good, involving relaxed foreign merchant restrictions:
But in a very few years, the king, and the great men, so sensibly felt the evil consequences of expelling the merchants strangers, that they thought it necessary to recall them: and then the City of London began their complaints, but met with an absolute repulse: For experience had taught the king and the parliament to distinguish between the general interests and welfare of the kingdom, and the monopolizing views of individuals. This prince proceeded further, and in the thirty-first year of his reign, granted a charter of protection to merchants strangers, which contained considerable privileges, when compared to the difficulties they had formerly laboured under.
(The prince in question is Edward I, and the charter is the Carta Mercatoria of 1303.)
Tucker positively cites a reply to Edward II's request for ceasing Scottish trade that was purportedly given by Robert III, Count of Flanders, saying that Flanders is "common to all the world, where every person finds a free admission," that "this privilege from persons concerned in commerce [cannot be taken] without bringing ruin and desolation," and that trade is simply a neutral engagement in trafficking wares without partaking in whatever errors of the trading country. Tucker adds that these maxims "were too large and noble to be understood by an ignorant, or relished by a prejudice people."
An old way of policing rowdy kin groups was some form of collective suretyship, as for instance frankpledge in Norman England. You don't need such subject populations to be "loyal to national institutions" or "assimilated" to any strong degree; simply that you get a smart fraction to serve as brokers to the authorities, and have various amercements be distributed communally, not individually. In all likelihood there will be a natural tendency to endogamy and segregation via geographic clustering.
In any event, there are far too many geopolitical factors, push and pull, that incentivize governing coalitions to court aliens for various purposes. Blithely assuming there aren't is simply avoiding the issue.
The French Revolution produced a substantial migrant crisis of its own. With it, plenty of opportunity. Emigre armies were used by countries fighting the militant French Republic -- the most notable and long-lasting of them being the Army of Condé, fighting alongside Austria.
In 1797, Trophime-Gérard, marquis de Lally-Tollendal (1751-1830), a former monarchien/constitutional monarchist, publishes an "address to the French people" in defense of the royalist emigres, fighting the Jacobin line of the emigre-as-counterrevolutionary-spy.
Much of it deals with the confiscated noble estates and the consequences thereof. One of the more interesting things he tries to do is attempt to appeal to the more illiberal sides of both classical and Enlightenment republicanism (p.324-8):
French people, I arrive at another capital truth, to which I will call more than ever all the faculties of your attention. It is a fundamental basis equally necessary to public morality, that this morality itself is indispensable for the magistrates of the people and for a republican state.
"Remove religion from man," said Cicero, "and his life is only troubled, his institutions are nothing but disorder." "Remove piety from the gods," he said again, "and that virtue which is universal, that virtue which is the virtue par excellence, justice, will disappear with it."
Do you like better to consult your cherished oracles? Ask Mably: he will tell you that he does not believe in the empire of good manners, where only men will be appointed for magistrates; where the magistrates and the citizens will not be persuaded of this important truth of a providence which governs the world, which sees the most secret motions of our soul, which will punish vice and reward virtue in another life. Ask Jean-Jacques. At the very moment when he opens a field without bounds to the wills, the whims of the multitude, Rousseau, fanatic in both directions, and not knowing how to do so that the force of the brake is due to the excess of license, not only wants a civil religion that always places man in the presence of this important truth; not only wants the sovereign to impose on each individual a profession of faith, but he declares whoever does not believe them, incapable of being a good citizen, nor a faithful subject; he condemns him to banishment.
The Legislative Assembly being the tempestuous body it was, Lally-Tollendal quotes the Girondist deputy Pierre Victurnien Vergniaud from Aug 8, 1792 about the legality of the emigres' actions: "If you have seized the property of the emigrants, it is not because they have abandoned their country. If they had had no other intention than to adopt another country, you would have made them, like the other foreigners who had been sacked in France, enjoy the protection of your laws. But they emigrated to form conspiracies, to incite enemies to the country they fled when she was in danger. They armed themselves to tear her breast. From then on, the law could only be invoked against them to punish them ..."
This statement is used to support the position that only emigres engaging in foreign plots can permissibly be expropriated.
All of this, however, ties to a wider defense of the landed gentry in general. (p.249-53) "The obvious, necessary, unique interest of the landowner is freedom, justice, order, peace, without which his harvest, his land, his house will be plundered, ravaged, demolished," he says.
"Because everything, even the passions of the landowner, attaches to order; for if he is ambitious and wishes to buy votes, he must receive his revenues; if he is greedy and wishes to increase his fortune, he must fertilize his land; now, to perceive as to increase his products, he needs public tranquility."
"Because, on the contrary, the proletarian who has nothing is more easily inclined to desire troubles which, without exposing him to any loss, offer him chances of gain; and the cosmopolitan, of whom all wealth is mobile, is less embarrassed to dig the abyss of public misery, to swell his peculiar treasure, which he will carry in triumph to the neighboring nations the day his nominal fatherland will sink into shame and disaster of bankruptcy."
"Because if, in the absence of personal passions, it is a question of becoming the instrument of foreign excesses, we still find that corruption attaches itself to need, seduction to ignorance, incorruptibility to independence and lights, independence and lights to the property."
And of property census suffrage: "Because, finally, if the examples are not superfluous where the principle is so evident, the assent of all the free peoples has in practice devoted this unassailable theory; that in the Roman republic even a patrician was excluded from the senate, if he ceased to have the income prescribed for entry; that, in the democratic Athens, the tribes were classified by reason of their properties, and the political magistratures exclusively devolved upon the proprietors; that the wise, the opulent Carthage was modeled on the same rule; that today no one can be a member of either the British parliament or the states of the Empire without land ownership. Should we say everything? Should we crown so many lessons by the most terrible of all?"
In consequence: "Now property, especially territorial property, does not exist in France; what has been put in its place by usurping the name and destroying its essence. As a false homeland had been substituted for the true, that is to say, a patrician homeland for a mother country, so a false property has been substituted for the true, and that of today has precisely all the dangers. opposed to the benefits of the other; that is to say that you have, instead of a property which reassures and ennobles, a property which disturbs and degrades; instead of a property which preserves and fertilizes, a property which destroys and dries up; instead of a property which guarantees at once both morality and independence, a property which necessarily supposes either immorality or servitude; lastly, instead of a property which desires order, which cherishes and succours the government, a property which, even by calling the order, disturbs it; who, even if born of the injustice of the government, or rather because it was born of it, despises it, dreads it, cheats it, and forsakes it."
Well, with all of these landed men being chased out of their homeland by mobs, there you have a humanitarian crisis that many states would be glad to participate in alleviating. Besides, major threats to national order have a supranational incentive for their being vanquished if they spill over.