All the characters in The Things They Carried are haunted by guilt and are looking for someone to blame. They feel guilty for the deaths of men in their platoons, for the deaths of the Vietnamese, and for their own inadequacies. Their reasons for this guilt can be as valid as being the one who accidentally drew mortar fire down on the platoon, or as irrational as cracking jokes about the dead. When they look for people to blame, they blame on a grand level (the war, the American voters, the Viet Cong) and on an individual level (a medic who treats Tim's wound badly, a soldier's own failure to select a good spot to set up for the night).
The characters in The Things They Carried live in a world of perpetual blame and guilt. Blame compounds guilt until the characters are so destroyed that they're unable to deal with civilian life.
O'Brien complicates the idea of culpability in war by showing us that we can blame any one person, country, or idea for a war. But to do so would be a dangerous oversimplification.