US President Joe Biden said Wednesday he plans to send 31 M1 Abrams tanks to Ukraine, reversing his longstanding resistance to Kyiv's requests for the highly sophisticated but maintenance-heavy vehicles.

His announcement came after Germany said it would send 14 Leopard 2 tanks to Kyiv from its own stock, ending an apparent standoff that saw Berlin officials indicate they would only send tanks if the US did too.

Here's what you need to know:

• Why it's significant: NATO's secretary general said the tanks "will significantly strengthen" Kyiv's combat capabilities. Ukraine's President Volodymyr Zelensky has consistently asked Western allies for modern tanks as Kyiv prepares for an expected major Russian counteroffensive in the spring.
• It's not just the US and Germany: Other countries, especially those with German-made tanks, have also announced contributions. CNN cannot confirm the total number of Leopard 2 tanks to be delivered, but pledges made by multiple nations mean Ukraine is in line to receive dozens of them. Germany’s main governing party said Wednesday that Western allies will send around 80 Leopard 2 tanks in total.
• Why send tanks now: The donations will provide Kyiv’s forces with a modern and powerful military vehicle and come as a blow to the Kremlin, which has seen a growing campaign to equip Ukrainian troops with high-tech fighting systems as Russia’s war nears the one-year mark.
• But don't expect an instant impact: Speaking before Biden's announcement, senior US officials framed the decision as an investment in Ukraine’s “longer term capabilities,” an indication the US sees the war extending well into the future. Ukraine hopes the new tanks can help it retake territory seized by Russia, including in the Donbas. That could also include Crimea, the peninsula annexed by Russia in 2014.
• Possible challenges: The Abrams tanks will take months to arrive, senior US officials said, and will require extensive training for Ukrainian troops on how to operate and service them. The US must also navigate complicated supply chains for the components required for the tanks. The procurement process will take months, the US officials said, though Germany's Leopards will arrive in the nearer term. Portugal's foreign minister said it will take two to three months before Leopard 2 tanks are fully operational in Ukraine.
• What happens now: In the meantime, the US will begin a "comprehensive training program" for the Ukrainians on the Abrams, which will occur outside Ukraine.