Donald Trump has made opposition to immigration the signature policy of his presidency, and used anti-immigration fervor to animate his supporters and fuel a political movement. FRONTLINE reveals the inside story of how an unlikely group of outsiders became the driving force behind Trump’s most controversial initiatives, as part of a grand strategy they would call “zero tolerance.”

Drawing on nearly 40 lengthy, on-camera interviews, this documentary describes how Steve Bannon, Stephen Miller and Jeff Sessions saw in the Trump candidacy a chance to roll back immigration—and how they have waged a war inside the White House to keep an unpredictable president on course. Zero Tolerance shows that while Trump rode to power amid a chorus of “Build the Wall,” behind the scenes the president was an unreliable ally on implementing key anti-immigration plans. As the president waffled, Miller, Sessions and Bannon played the central role in bringing their “imperfect instrument” back into line.

It’s a must-watch look at how America’s immigration policies—and the rhetoric and images surrounding them—reached this moment.