WASHINGTON — A white National Guard commander called the standoff in Lafayette Square “the Alamo,” implying that the White House was under siege. Black members of the D.C. Guard objected to turning on their neighbors. Army leaders told pilots to “flood the box with everything we have” as two helicopters buzzed protesters in the streets. The National Guard is now engaged in an investigation of the havoc a week ago Monday in downtown Washington, similar to after-the-fact examinations more common to battlefields in Iraq and Afghanistan. There will be questions, interviews and competing narratives.