Russia is at war with Ukraine. Here’s the background you need to know.

People cross on an improvised path under a bridge that was destroyed by a Russian airstrike, while fleeing the town of Irpin, Ukraine, on March 5. (Vadim Ghirda/AP)

For weeks, Western officials warned that a massive buildup of Russian troops on the border with Ukraine signaled a plan by Moscow to invade. Now, a full-scale invasion by Russian forces is underway, with missiles and artillery raining down on major cities, hundreds of civilians killed and some places — including Ukraine’s largest nuclear power plant — already under Russia’s control.

The war has produced more than 1.3 million refugees since it began on Feb. 24, according to the United Nations refugee agency, and is upending decades of peace in Europe.

Russian President Vladimir Putin has said that he seeks to “demilitarize” Ukraine and ensure that it stays out of NATO, whose expansion he sees as a security threat to Russia. But the invasion has already come at a steep cost to Moscow, now the target of sweeping international sanctions that have crashed Russia’s currency and sparked an exodus of foreign investors and brands from the country.

Russia boycott: A list of global campaigns that are underway in support of Ukraine

Here’s what we know about why Russia has attacked Ukraine — and what could come next.

The Washington Post's Sudarsan Raghavan reports from Markhalivka, Ukraine, where at least six people died from a suspected Russian airstrike. (Video: The Washington Post)