SEVASTOPOL, Ukraine  A year after its war with Georgia, Russia is engaging in an increasingly hostile standoff with another pro-Western neighbor, Ukraine.

Relations between the two countries are more troubled than at any time since the Soviet collapse, as both sides resort to provocations and recriminations. And it is here on the Crimean Peninsula, home to a Russian naval base, where the tensions are perhaps most in danger of bursting into open conflict.

Late last month, the Ukrainian police briefly detained Russian military personnel who were driving truckloads of missiles through this port city, as if they were smugglers who had come ashore with a haul of contraband. Local officials, it seemed, were seeking to make clear that this was no longer friendly terrain.

Ukraine has in recent years been at the forefront of the effort by some former Soviet republics to switch their alliances to the West, and it appears that the Kremlin has, in some sense, had enough.