The most recent raft of U.S. military aid to Ukraine contained weapons specifically designed for the new phase of the war with Russia in the Donbas region.

Over the past two weeks, the Biden administration has committed more than $1.5 billion in aid that included howitzers, Phoenix Ghost tactical unmanned aerial systems, and more. They are now prioritizing different types of equipment given the Russian military's new focus on the region in eastern Ukraine.

"You’ve heard us talk or say in the past, recent past, the nature of the fight has evolved because the terrain that they’re now focused on is a different type of terrain. So they need long-range fires," Defense Secretary Lloyd Austin said at a press conference in Poland on Monday. "You’ve heard them express a need for tanks. And we are doing everything that we can to get them the types of support, the types of artillery and munitions that will be effective in this stage of the fight."

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Russian forces regrouped, and many have redeployed to the Donbas, where there is a significant pro-Russian separatist presence and fighting between the two sides has persisted since 2014. Secondarily, the Russians looking to capture the territory along the southeastern coast of Ukraine that would provide them with a land corridor from Crimea, which they annexed in 2014, and the Donbas.

“It’s not forest. It’s not urban,” retired Rear Adm. Mark Montgomery, a senior fellow for the Foundation for Defense of Democracies, told the Washington Examiner of the Donbas region. “It’s lower population density.”

The United States has provided Ukrainian forces with more than 1,400 Stinger anti-aircraft systems, 5,500 Javelin anti-armor systems, 14,000 other anti-armor systems, 700 Switchblade tactical unmanned aerial systems, 90 155 mm howitzers, 183,000 155 mm artillery rounds, 16 Mi-17 helicopters, and hundreds of Humvees, among other weapons.

The U.S. has trained and will continue to teach limited numbers of Ukrainian soldiers on various weapons they're providing, with the intention that they will be able to return to Ukraine with the knowledge and ability to train other soldiers. The Ukrainians have been trained on the aforementioned Phoenix Ghosts, which are similar to the switchblade drones that attack targets, as well as the howitzers.

The latest aid has "been sent with the knowledge that the first Russian campaign was over and a new Russian campaign in the east and maybe in the south was beginning. And the Department of Defense altered the tools they're sending, the equipment they're sending, to account for that," Montgomery said.

Austin and Secretary of State Antony Blinken traveled to the capital of Ukraine, Kyiv, on Monday to meet with Ukrainian President Volodymyr Zelensky. They became the first Biden Cabinet officials to visit the country since Russia's invasion began Feb. 24, though leaders from other countries have shown their support in person. Officials have begun visiting Kyiv now that Russia no longer has troops actively trying to topple the capital, failing to do so in the first couple weeks of the invasion.

The Ukrainian forces face a difficult challenge in defending the Donbas because Russia is “closer to its home bases” and is more "familiar” with the area, compared to its early efforts to topple Kyiv, Montgomery explained, referencing the fighting in the Donbas that dates back years.

Rustam Minnekayev, a senior Russian commander, claimed Friday that Moscow now wants "full control" of southern Ukraine in addition to the Donbas region. Russia increased its troop presence in the Donbas region in the form of additional battalion tactical groups, which brings the total number of BTGs in the area to roughly 80, a senior U.S. defense official told reporters last week, and each one consists of roughly 800-1,000 service members.

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Also last week, a senior U.S. defense official told reporters that the Pentagon believes Russia still has roughly 75% of its capabilities that they had at the beginning of the invasion, while Pentagon spokesperson John Kirby said Friday that they "don't anticipate that it's going to be over, you know, in days or even just coming weeks."

U.K. Defense Secretary Ben Wallace said Monday that an estimated 15,000 Russian troops had died of the more than 160,000 amassed at Ukraine's border.

Michael Kofman, the director of the Russia Studies Program at CNA, estimates that Russia has one “last major offensive” left given their losses unless they mobilize their forces nationally. Regardless of how the fighting plays out, which so far has resulted in limited gains for either side, the “Russian military will be largely exhausted in terms of offensive potential.”