SEVASTOPOL, Crimea — Surrendering to Russia's inexorable seizure of Crimea, Ukraine announced plans Wednesday for mass troop withdrawals from the strategic peninsula as Moscow-loyal forces seized control of Kiev's naval headquarters here and detained its commander.
Attempting to face down the unblinking incursion, Ukraine said it would hold joint military exercises with the United States and Britain.
Hours after masked Russian-speaking troops forced their way onto Ukraine's main naval base here, forlorn Ukrainian soldiers streamed out carrying clothing and other belongings in bags. A group of local militia and Cossacks, later joined by officers from Russia's Black Sea Fleet, looked on.
Just how many retreating troops Ukraine will have to absorb in what amounts to a military surrender of Crimea was unclear. Many servicemen have already switched sides to Russia, but authorities said they were prepared to relocate as many as 25,000 soldiers and their families to the Ukrainian mainland.
Humbled but defiant, Ukraine lashed out symbolically at Russia by declaring its intent to leave the Moscow-dominated Commonwealth of Independent States, a loose alliance of 11 former Soviet nations. The last nation to leave the group was Georgia, which lost a brief war with neighboring Russia in 2008 and ended up losing two separatist territories.
Vice President Joe Biden, in Lithuania trying to reassure nations bordering Russia alarmed by the sight of an expansion-minded neighbor, said the United States would stand by them.
"We're in this with you, together," Biden said.
Ukraine has been powerless to prevent Russian troops from taking control of Crimea, which President Vladimir Putin formally annexed Tuesday with the stroke of a pen. Crimea's absorption came after a hastily organized referendum in which the population overwhelmingly, albeit under conditions akin to martial law, voted in favor of seceding from Ukraine and joining Russia.
Russia's Constitutional Court chairman, Valery Zorkin, said Wednesday the treaty signed by Putin has been ruled valid, meaning it now only requires ratification by the Russian parliament.
On Wednesday morning, militiamen under apparent Russian command barged their way into Ukraine's naval headquarters in Sevastopol, detaining the head of Ukraine's navy and seizing the facility. The incursion, which Ukraine's Defense Ministry described as being led by a self-described local defense force, Cossacks and "aggressive women," proceeded with no resistance.
Upon gaining entrance to the base, the storming party raised a Russian flag on the headquarters square.
The unarmed militiamen waited for an hour on the square and, after the arrival of the commander of the Russian Black Sea Fleet, they took over the building.
By afternoon, they were in full control of the naval headquarters.
Meanwhile, in a warning to Moscow, Biden declared that the United States will respond to any aggression against its NATO allies, including neighbors to Russia.
Standing with two Baltic leaders in the Lithuanian capital, Vilnius, Biden said the United States was "absolutely committed" to defending its allies, adding that President Barack Obama plans to seek concrete commitments from NATO members to ensure the alliance can safeguard its collective security.
"Russia cannot escape the fact that the world is changing and rejecting outright their behavior," Biden said after meeting with Lithuanian President Dalia Grybauskaite and Latvian President Andris Berzins.