An uprising in Iraq is the broadest in decades. It’s posing an alarming threat to Baghdad and Tehran.

BAGHDAD — From Baghdad to the Shiite Muslim shrine city of Karbala and farther south, Iraqis are pushing for a revolution. They fill central squares to sing and dance from daybreak, and face down riot police when night falls.

Iraq’s streets are no stranger to power struggles. They’ve been a stage for sectarian conflict and for the Islamic State’s emergence. But the crowds are different this time, and so is the threat now posed by the largest grass-roots movement in Iraq’s modern history: A new generation raised in the shadow of the U.S.-led invasion is rising, and politicians from Baghdad to Tehran have been caught on the back foot.