The race to prepare for the battle of the Donbas is on, with Russia shifting its troops, materials and objectives to the eastern region of Ukraine, and the U.S. and its allies rushing in weaponry to equip Ukraine’s forces.

The big picture: In the first phase of the war, Russian troops attacked across three broad fronts hoping to quickly take cities, but they became overstretched and suffered heavy losses. Russia is pulling tens of thousands of those troops back and preparing them for a narrower and potentially decisive fight for the Donbas.

• Russian President Vladimir Putin has said that peace talks are at a dead end, and Moscow’s focus will now be the “liberation” of all of the Donbas, about one-third of which was controlled by Russian-backed separatists prior to his Feb. 24 invasion.
• Putin justified the invasion in part through erroneous claims of a “genocide” against Russian speakers in the Donbas — a claim undermined by the fierce resistance put up by civilians in largely Russian-speaking cities like Mariupol.
• That city, the third-largest in the Donbas after Russian-held Donetsk and Luhansk, could be within days of falling to Russia after seven weeks of brutal bombardment. That would give Putin a much-needed symbolic victory and free up more Russian forces for the coming offensive.

The battle for the Donbas will remove some of the obstacles Russian forces struggled with elsewhere in the campaign.

• It’s largely flat and wide open, unlike the more urban terrain near Kyiv. That’s ideal for Russian artillery and tanks, though rains have made much of the ground muddy.
• Russia should have fewer supply line issues fighting so close to its own territory, and fewer organizational and communications challenges in a more concentrated fight.

Yes, but: Manpower and morale are both obstacles. It will take time to properly reorganize and redeploy so many units given the heavy losses Russia has suffered, says Michael Kofman, a top expert on Russia’s military at CNA. Large numbers of reserves aren’t expected anytime soon.

• Ukrainian forces have also dug in across much of the region through eight years of war, and they'll likely be fortifying towns and laying mines to further slow the Russian advance, he says.
• Kofman thinks Russia’s commanders would be wise to delay their offensive until they’ve had time to fully build up their force, rather than “repeat the errors from earlier in the war.”
• But that will also leave time for Ukraine’s Western partners to supply additional heavy weaponry.

What to watch: Concerns in Washington and some European capitals that Moscow would view shipments of certain offensive weapons as overly “provocative” have largely faded, Axios’ Zachary Basu reports.

• The $800 million package President Biden announced Wednesday includes helicopters, artillery systems and armored vehicles.
• Ukrainian President Volodymyr Zelensky pressed for more in his latest video address, saying, "Freedom must be armed better than tyranny."

The latest: Russia also suffered a major blow Thursday on the 50th day of war: the Moskva cruiser, the flagship of its Black Sea fleet, has sunk. It's the largest military vessel to sink in decades.

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