If you want to go to war, you (and your political party) need to be in control of the senate. WHen it comes time to hold an election, the people will vote on what political party they want to see in power. If they're happy (Overall Morale > 50) you'll most likely stay in power (though I suggest keeping morale around 70 for saftys sake). If you morale gets too low, your government party will get the boot from office, and your opposition party will take the lead, and when you try to go to war they'll vote against it.
In the end, to go to war you have to wait for the next election, get your people happy enough to vote your party back into power, then try to go to war.
Hope this helps.