I understand your question, but it's a lot more complex than you are making out.
It will not simply be a matter of "whoever gets there first owns the planet", and war does not necessarily follow dispute.
It is unlikely that any nation currently has - or will ever have - the financial capacity to colonise another planet. Any such undertaking will need to be a joint effort which brings its own implications for treaty, relations and economics.
Nobody goes to war on a whim. Whether or not a government will declare war to defend an interest depends mainly on three things:
1) The policies of the party in power,
2) The mood of the population,
3) The cost-gain difference.
The first two are almost impossible to predict further ahead than five years in most countries. The third one depends on the specifics of the interest in question, and really can't be just guessed.