Government shutdowns are a useful window into what really matters to politicians. It’s one thing to say you care about an issue. It’s another to care about it enough to tell hundreds of thousands of federal employees to stay home, and to risk the political blowback that arises when their absence starts wreaking havoc on ordinary Americans’ lives.

So it’s noteworthy that, according to Vox’s Dylan Matthews, the federal government has shut down because of disputes about domestic spending (1976, 1981, 1982, 1983, 1990, 1995, 1996, 2013), abortion (1977, 1979), military spending (1978, 1982, 1983), civil rights (1983, 1984), crime (1984), welfare (1986), foreign policy (1987), and because congressional leaders just plain forgot to pass legislation necessary to keep it open (1982). But until last weekend, it had never shut down because of a dispute over immigration.

It’s more evidence that immigration has become far more central to American politics than it was a generation ago. (Quick: try recalling Bill Clinton or George H.W. Bush’s position on the issue). It’s also become far more polarizing. As recently as a decade ago, when they ran against each other for president, Barack Obama and John McCain mostly agreed on the subject.

What explains this? Part of the answer is obvious: The United States contains a lot more immigrants. In 1960, when America was, presumably, great, the foreign-born comprised less than five percent of America’s population. Now they comprise more than 13 percent. Moreover, today’s immigrants are now mostly people of color. In the 1960s, when US immigration law strongly favored Europeans, the country that sent the most people to live in the United States was Italy, followed by Germany and Canada. In 2015, more than a quarter of those who immigrated to the U.S. were from Mexico. So immigration is changing America’s racial and ethnic character.

But while this explains why immigration was likely to become a more important issue over time, it doesn’t explain its dramatic rise in political significance over the last few years. In recent years, according to the Pew Research Center, immigration has actually slowed, and there are fewer undocumented immigrantsin the US than there were in 2009. So why now? One of the explanations is probably Barack Obama. The fact that the Democratic Party—which gets an increasing share of its votes from immigrants and people of color—elected the son of a visiting Kenyan college student as president probably elevated immigration’s political salience, and exacerbated the partisan divide surrounding it.

But there’s another factor, which often gets overlooked. Relative to the rest of the 21st century, this is a period of prosperity and peace. The deficit has dropped by almost two-thirds since 2011. The unemployment rate has been cut in half. Wages for middle-class Americans are even starting to rise. Wartime casualties are down too. In 2010, 559 Americans died fighting in Afghanistan and Iraq. Last year, it was 32.

Why has this made immigration more central? Because politicians can afford to dwell on it. When the country is in recession and deficits are rising—as in the early 1990s and the late 2000s—debates over taxing and spending tend to dominate Washington. When large numbers of Americans are dying in war—as occurred in the mid-2000s—politicians focus on that. But when neither is taking place, and there’s more room for politicians to shape the conversation, questions of American identity often come to the fore.

This political moment doesn’t feel like the late 1990s. It feels unprecedented because America’s president is Donald Trump. But there are similarities. It was in the late 1990s, after the Cold War, after the Gulf War, and after America emerged from recession, that the Republican Party launched an effort to impeach Bill Clinton that centered, in large measure, on expelling the moral toxins that the 1960s had supposedly injected into American life. It’s not entirely coincidental that the #MeToo movement has sparked a reconsideration of the Clinton impeachment now, at another moment when debates over war and deficits have died down.

Trump—with his allies at Fox News, Breitbart and elsewhere—has taken advantage of this relative political vacuum to focus on immigration. So have his progressive opponents. Right now, it’s what politicians in both parties truly care about. And that may well remain the case until a stock-market crash, a recession, or another war—at least one of which, history suggests, is likely in Trump’s first term.