Alaska, any state attempting to invade it would need to commit an act of war against either the Commonwealth (if attempting a land invasion) or the CIS (if attempting a navel or airborne invasion). Unless they were to form an alliance with either of these groups. I couldn't see the Commonwealth buddying up to anyone unless they'd already taken the rest of the US, and I think the CIS would only pair up with Alaska, in an attempt to turn it into another member nation.
Besides, whoever wins, only real winner would be the Commonwealth (oh, hey, new US, we're making Canada bigger, 'kay? =D ), the CIS (Oh, preved, new US, sign this treaty giving us absolute control over the Bering strait or we nuke you to oblivion, da? =D ) or Mexico (oh, hola, new US, we'll be invading you now, ci? =D )