You forget history Ukraine, used to rule what is now Russia.
They gave that up.
Why are they a threat?
Great Northern War 1709, Charles XII of the Swedish Empire invades Russia through Ukraine, is defeated at Poltava (between Kiev and Kharkov)
Napolean's foreign minister in 1811 Jean-Baptiste de Champagny, devised a plan to follow Charles XII's route to invade Russia through Ukraine, due to better climate, more food stores, and he hoped, with the help of Black Sea Cossacks. Napoleon fired de Champagny in 1811 and instead decided to attack Alexander I's army where it was presently located, in Belarus.
WWI: As part of Treaty of Brest-Litovsk Germany required (now Soviet) Russia to accept Ukraine independence, and Ukraine immediately became part of the Axis Powers. German troops invaded/occupied all the way to Rostov-on-Don
WWII: 1941 German Army Group South invades Ukraine, 1942 Operation Blue German Army Group South attacked through Ukraine to the Caucuses and Stalingrad.
the lesson of history is that all European empires (of which the USA is now one) will always use Ukraine as tool against Russia, and the most logical route for destabilization or invasion of Russia.
The other lesson is that its nearly always failed. The US neocons are idiots to expend US power on this.