As Russia refocuses its invasion of Ukraine on the east, recognition is growing in Kyiv and allied capitals that the window to prevent the nation’s partition and a long war of attrition may be narrow.

The recent withdrawal of Russian troops from around Kyiv represents a defeat, after Ukraine’s military stalled their advance with a combination of urban warfare and attacks on supply lines.

Yet to roll back, or even contain a grinding advance by reinforced Russian units across the eastern Donetsk and Luhansk regions would mean taking the fight to open battlefields, requiring more than just the light anti-tank and anti-aircraft missiles the U.S. and Europe have supplied so far.