This is a matter of personal freedom. A citizen may leave his state and the things he gets involved in another country are none of the business of his government.
If a government sends its military personnel on a mission abroad, that's a completely different case.
Hmm, yes, now that I see it your way, Russian government has no right to stop thousands of soldiers and columns of Russian tanks (built in average citizen's backyard) from invading a foreign country. But, since Russian government is so helpless to control their own border crossings and people (Russia can only rape and kill so many of its own people at a time, we cannot expect too much) really nothing that Czar Putin can do. Time to hit the vodka and ignore this mess! Ukraine isn't a real country anyway. /s