Why didnt O Brien stay out of the war?

The author himself explains his reasons for going to war in one simple word..... embarrassment.

All those eyes on me—the town, the whole universe—and I couldn't risk the embarrassment. It was as if there were an audience to my life, that swirl of faces along the river, and in my head I could hear people screaming at me. Traitor! they yelled. Turncoat! Pussy! I felt myself blush. I couldn't tolerate it. I couldn't endure the mockery, or the disgrace, or the patriotic ridicule. Even in my imagination, the shore just twenty yards away, I couldn't make myself be brave. It had nothing to do with morality. Embarrassment, that's all it was.