DOHA, Qatar — The United States military is installing a new command center at a heavily guarded base in this small Persian Gulf state that would be ready to serve as the main headquarters for a war on Iraq. The official purpose of the work at the base, As Sayliyah, is to prepare for a major American military exercise in December called Internal Look. But it will be no ordinary exercise. American officials say that it will be the first time that a war game of its type has been conducted outside the United States and that the command and...