German military doctrine under the Nazi regime, characterized with some controversy as Blitzkrieg, called for air strikes that softened an intended victim for attack by motorized, mechanized, and airborne forces on the schwerpunkt (focal point), followed by encirclement by motorized forces, and exploitation of the gap by conventional infantry forces. Radio communication allowed for the close coordination necessary for such attacks, and allowed for coordination of the air force. The Nazis as much broke the rules of engagement which previously governed nations at war (such violations often deemed after the war as crimes against peace) as they innovated techniques of war. Axis reverses beginning with Allied routs of overextended German forces in El Alamein and Stalingrad resulted from British and Soviet forces adopting Nazi field strategies, and as the United States became a participant in the war it adopted much the same techniques of aerial attack upon Nazi Germany, if with greater force than the Luftwaffe could ever inflict.