It was an ironic moment. On Friday, President Donald Trump signed an executive order barring immigrants and refugees from seven nations--Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen--from entering the United States. Among other effects, the ban created chaos at airports around the nation where visitors and refugees from the affected countries, carrying visas or green cards which until that moment would have granted them legal status, disembarked from their planes into a country they were no longer allowed to enter.

Meanwhile, a couple of hundred miles away from the White House, Warren Buffett and Bill Gates were addressing students at Columbia University on a range of topics that included immigration. Though neither he nor the audience were aware of Trump's new executive order, Buffett explained why he's pro-immigration. "Well, immigration, this country is built on it. I always say to people that are anti-immigration, 'let's put it in retroactively.' And everybody leaves," he said. The students laughed in response.

"This country has been blessed by immigrants," he added later in the event. "And you can take them from any country you want and they've come here and they've found something that's unleashed the potential that the place where they left did not, and we're the product of it."

Only Christians allowed.

Trump's executive order has one exception, allowing entry only for Christian immigrants from the disallowed countries. So it's worth repeating Buffett's favorite example of the effect immigrants have had on the U.S., one he shared with the Columbia students: "If you think about it," he said, "we are sitting here, in part, because of two Jewish immigrants who in 1939 in August signed the most important letter perhaps in the history of the United States." The letter in question was from Albert Einstein and Leo Szilard. It warned President Franklin D. Roosevelt that Germany was likely trying to build an atomic bomb and recommended that the U.S. government also set to work on developing nuclear capabilities. "If it hadn't been for those two immigrants, who knows whether we'd be sitting in this room," Buffett said.

Buffett, of course, is not alone in objecting to Trump's anti-immigration policies. Apple CEO Tim Cook, Facebook CEO Mark Zuckerberg, Microsoft CEO Satya Nadella, and Google CEO Sundar Pichai all voiced opposition to the ban, warning that it would negatively affect their companies' operations. Microsoft took the added step of formally warning investors that the ban might have an impact on its financial performance because it would create staffing difficulties. Meanwhile, Google called for any traveling employees from the affected nations with green cards or H-1B visas to return to the U.S. immediately, and for those already here not to leave the country.

As protesters gathered at JFK and other airports, at least two federal judges, one in Brooklyn and one in Washington, have issued orders blocking the deportation of immigrants who have legally obtained visas or green cards. Many immigrants have been detained for hours by officials who aren't sure what to do with them, while the ban faces legal challenges. What happens next is anyone's guess, but for now, anyone born in the affected countries who does not already have full U.S. citizenship stands on very shaky ground.