By Christine Clarridge Seattle Times staff reporter Big as the Washington Monument, as complicated as the space shuttle and as stealthy as its black silhouette suggests, the nation's Trident submarine is the ultimate doomsday machine. But in the long thaw of the Cold War, when regional conflict is a more pressing threat than nuclear annihilation, the Navy is beginning to redefine the Trident's mission. After Sept. 11, a jittery Congress approved a defense-spending bill that included nearly $4 billion to "transform" the nation's four oldest Trident submarines into modern street fighters. Two will sail from the Navy's base at Bangor,...