In the face of a Democratic, media, and public outcry, President Trump last week ended his policy of separating children from adult illegal immigrants claiming asylum. It was his second step toward a sensible border policy. The first had been the initial decision to enforce immigration law and prosecute people sneaking into the country.

It was telling to watch how Democrats and left-wing commentators responded. Having shed tears all week about family separation and traumatized children, Trump's opponents began objecting to any detention of families entering the U.S. illegally. A chorus of critics described the new policy as "handcuffs for all." An MSNBC host portrayed as a dark “totalitarian” act the very idea of a border guard stopping and arresting an illegal immigrant.

Behind these objections is a premise that goes unstated because it is radical and deeply unpopular. It is that border enforcement is itself illegitimate. The notion that we would police who enters our country strikes some as inherently oppressive. Many of our elites, Left and Right, see borders as archaic and arbitrary. This is often related to their doubt about the need for, and morality of, the nation-state.

The unpopularity of this elite ideology has become obvious in elections here and in Europe, where the push to dissolve borders and nations is far more progressed. The ideology is as dangerous as it is unpopular.

Borders are inextricable from self-determination. People are not made to be solitary creatures, or even to live merely in family units. We are political animals, and we have access to our highest potentials only when we live in community with one another. Becoming part of a community is forming an identity not merely as “me,” but also as “us.”

For “us” to have meaning, it has to be defined. At worst, "us" is defined along class lines or racial lines. At best, it’s more open and diverse, like the United States of America. But “us” can never mean “whoever happens to show up.” Unless a community can control its membership — freedom of association is at the core of our Constitution — it can’t be a community. The nature of democracy and society prohibit that. A society has rules, and in a democracy, the rules are determined by the people. If an ever-shifting mob makes up “the people,” there are no consistent rules, and thus no society, no community.

This doesn’t mean a community or a nation should forbid outsiders. Communities and nations that seal themselves off tend to wither and die. Compassion also requires that we take in strangers in need.

As a people, however, we must maintain our right to control our membership. It is therefore a natural function of government to police our borders. The U.S. is the greatest country in the world. Nearly 7 billion people live outside the U.S., and most of them would probably prefer to live here if they could. We cannot take them all. That requires us to have standards, and to approve some and not others.

So, we can’t, or rather mustn't, simply stop enforcing immigration laws, and we can’t say come all who wish.

[Opinion: Ignore the open-border establishment — President Trump is right on immigration]

On the flip side, it would be cruel and imprudent to enforce these laws unnecessarily harshly. Just as we don’t jail speeders, and just as we don’t place cameras on every street corner to deter crime, prudence requires us to have a tolerance above zero for illegal entry or for overstay. Compassion requires us to treat illegal entrants humanely. That includes trying to keep families together.

Balancing these competing interests is difficult, and it is rightly the role of both the political process (Congress) and our executive branch. But in this balancing act, we cannot weight borders and national self-determination as nothing.

Without borders and border enforcement, there is no clear definition of the polity. Democracy cannot exist without a clearly defined polity. That’s why the European project of replacing nation-states with a continent-wide super-state, with porous borders both internally and externally, has been so undemocratic. Elites are fine with a shifting populous, because the public aren’t “us” in an important sense. The elite tribe, spanning borders, is their “us” that makes the rules.

Theoretically, we could give up on borders, as some seem to want. But that would mean giving up on having a democratic society.