The US command announced that it is sending 3,700 troops to Baghdad to try to quell the sectarian violence sweeping the capital, and a US official said more American soldiers would follow as the military gears up to take the streets from gunmen. The 172nd Stryker Brigade, which had been due to return home after a year in Iraq, will bring quick-moving, light-armored vehicles to patrol this sprawling city of six million people, hoping security forces respond faster to the tit-for-tat killings by Shi'ite militias and Sunni Arab insurgents. The US military hopes more armor will intimidate gunmen, who in recent weeks have become more brazen in their attacks. "This will place our most experienced unit with our most mobile and agile systems in support of our main effort," Gen. George W. Casey Jr., the top US commander in Iraq, said Saturday. "This gives us a potentially decisive capability to affect security in Baghdad." US President George W. Bush said this week that he had decided to send more troops to Baghdad after the surge in reprisal killings began to threaten the unity government of Prime Minister Nouri al-Maliki, which took power May 20.