BAGHDAD, Iraq - A growing number of senior American military officers in Iraq have concluded that there is no long-term military solution to an insurgency that has killed thousands of Iraqis and more than 1,300 U.S. troops during the past two years. Instead, officers say, the only way to end the guerilla war is through Iraqi politics - an arena that so far has been crippled by divisions between Shiite Muslims, whose coalition dominated the January elections, and Sunni Muslims, who are a minority in Iraq but form the base of support for the insurgency.