I don't think they were forced into the war. The whole country is fascist, (whether the population knows it or not) and see's Ozai as a protector, and a leader. The majority of the people are in fact, happy.
It's two generations, (or one if the person is a powerful bender) that have had all their history books rewritten and led to believe that the Fire Nation is doing some great justice to the world.
The war is actually a lot longer than 100 years. Because Roku had to deal with it's beginning in the prime of his life.