The incursion was the first time alliance planes had engaged enemy targets in NATO airspace. Poland’s leader called it “a large-scale provocation.”

Talks of a cease-fire in Ukraine have centered on the Donbas, a valued region for both sides and where most of the fighting is taking place. Michael Schwirtz, a global intelligence correspondent, and Anatoly Kurmanaev, a foreign correspondent covering Russia, break down who the Donbas is important to and why.

As negotiations over the war carry on, President Vladimir V. Putin is pushing hard with tactics that have evolved over three and a half years.