Former President Barack Obama offered a sharp criticism of the Trump administration’s family separation policy at the US-Mexico border on Wednesday, asking a “simple question” about watching children being separated from their fathers in real time.

“Are we a nation that accepts the cruelty of ripping children from their parents’ arms, or are we a nation that values families, and works to keep them together?” Obama wrote in a Facebook post honoring World Refugee Day. “Do we look away, or do we choose to see something of ourselves and our children?”

World Refugee Day is an international event meant to draw attention to the global refugee crisis.

This is the first time Obama is explicitly weighing in on the Trump administration’s policy, which has sparked outrage across the country. On Monday, he retweeted a tweet from his wife, former first lady Michelle Obama, agreeing with another former first lady, Laura Bush, about the “cruel” and “immoral” nature of splitting up immigrants and kids. “It breaks my heart,” Bush wrote on Twitter and in an op-ed in the Washington Post. (All four living former first ladies have condemned family separation.)

Sometimes truth transcends party. https://t.co/TeFM7NmNzU — Michelle Obama (@MichelleObama) June 18, 2018

Barack Obama on Wednesday reminded Americans that they’ve been “fortunate enough” to have been born in the United States and not to take that for granted. “Imagine for a moment if the circumstances had place you somewhere else,” he wrote. “Imagine if you’d been born in a country where you grew up fearing for your life, and eventually the lives of your children. A place where you finally found yourself so desperate to flee persecution, violence, and suffering that you’d be willing to travel thousands of miles under cover of darkness, enduring dangerous conditions, propelled forward by that very human impulse to create for our kids a better life.”

That’s the reality many of the migrants coming to the United States are facing, he wrote. He also harked back to America’s immigrant roots. “Almost all of us were strangers once, too,” he wrote.

A wave of activism and protests has swept across the country to try to put an end to Trump’s “zero tolerance” policy, which splits up families by charging parents with illegal entry into the US and sending them into criminal custody and treating their children as if they were “unaccompanied alien children” who had tried to enter the United States alone. Obama called for doing more than saying, “This isn’t who we are.”

“We have to prove it — through our policies, our laws, our actions, and our votes,” he wrote.

Obama’s full World Refugee Day statement is below.