When Russian troops invaded eastern Ukraine in 2014 and occupied the Crimean Peninsula, they faced little resistance from a decrepit Ukrainian military—inexperienced, hollowed out by decades of corruption, and lacking the most basic supplies, such as medical kits, boots, and proper helmets. Now, after years of reform and billions of dollars of security assistance from the United States, the Ukrainian military is battle-hardened and highly motivated after seven years of conflict with Russian-backed rebels in Ukraine’s separatist-held Donbass region.

“It’s certainly not going to be an easy operation. It’s not going to be a quick victorious war [for Russia],” said Andrii Zagorodniuk, who served as Ukraine’s defense minister from 2019 to 2020.

Since 2014, the United States has given $2.5 billion to support the development of the country’s armed forces, including Humvees, patrol boats, radar systems, and anti-tank Javelin missiles. But with Russian troops massing on Ukraine’s borders, U.S. officials are weighing requests by Kyiv for further assistance, including support to plug substantial gaps in the country’s air and naval defense capabilities, as Ukraine’s navy lost 70 percent of its vessels following the occupation of Crimea. This month, U.S. officials were dispatched to Kyiv to asess the country’s air defense needs, a Ukrainian defense official told Foreign Policy.