Have You Heard of The Budapest Memorandum?

In 1994, Bill Clinton, John Major (UK), Boris Yeltsin (Russia) and Leonid Kuchma (Ukraine) signed an agreement, called The Budapest Memorandum, promising to uphold the territorial integrity of Ukraine in return for Ukraine to give up its nuclear weapons.

There is more: in 2005, Senator Obama pushed bill that helped destroy more than 15,000 TONS of ammunition, 400,000 small arms and 1,000 anti-aircraft missiles in Ukraine.

That explains why White House was more or less accurate calling Russian invasion of Crimea an "Uncontested Arrival". Ukrainian army is simply disarmed.