Try it and Kiev is Russian in 2 months.
Do you think that they will be able to defend against the regular Russian army for 2 months? They were defeated by a bunch of coal miners and farmers in Donbass. IMO, everything will be over in a day. There will be only two major battles. The battle of Kiev will be fought in the morning and the battle of Lvov will be in the evening. Ukraine will be a part of Russia within 12 to 24 hours.
Realistically, if Russia made a move and Ukraine received no military support from the west, there would even be no battle. They'd likely just surrender, with any resistance moving underground, poking Russia from time to time with guerrilla type actions.
Alternatively, leaders would order the army to defend while jumping to the helicopters and fleeing across the Poland border themselves. But still, white flags will be up in the air in no time.
Actually it simply would look like in Crimea. 100% surrender and 70% even changing the side.
Thousands of former Ukrainian soldiers are actually now serving in Russian ranks in Crimea.
They just need to take out the leaders and radicals.