For much of the winter, the war in Ukraine settled into a slow-moving but exceedingly violent fight along a jagged 600-mile-long frontline in the southeast. Now, both Ukraine and Russia are poised to go on the offensive. They are looking for vulnerabilities, hoping to exploit gaps, and setting the stage for what Ukraine warns could be Moscow’s most ambitious campaign since the start of the war. Ukraine must now defend against the Russian assault without exhausting the resources it needs to mount an offensive of its own. President Vladimir V. Putin of Russia has given an order to take all of the Donbas region by March, Ukrainian intelligence says.

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