In Africa and Sicily Anglo-American forces had seen elements of a new kind of close combat that the German army had developed in Russia, Bevin Alexander explains (in How Hitler Could Have Won World War II), but on the boot of Italy they came firmly up against it:
The Germans saw in Russia that infantry actions were fought overwhelmingly at close range, 75 yards or less, and introduced the MP38 and MP40 “Schmeisser” machine pistol that fired high-velocity pistol bullets, giving heavy unaimed fire to blanket an area and suppress enemy resistance. The Russians introduced a different sort of weapon that achieved the same effect, the PPSh41 7.62-millimeter submachine gun (burp gun). Supported by fast-firing portable machine guns, the MG-34 and MG-42, the Schmeissers gave Germans mobility and high volume of fire. They never replaced all their standard medium-range bolt-action rifles (the Mauser Kar. 98k) or employed many of the next-generation automatic assault rifles (Sturmgewehr), but Schmeissers and MG-34s and MG-42s gave them high capacity to defend against attacks.
The British replaced in part their medium-range bolt-action rifle, the Enfield No. 4, with various submachine guns (“Sten guns”) that fired the same 9-millimeter pistol cartridge as the Schmeisser, coupling them with the Bren gun, a reliable light machine gun.
The Americans were slower to replace the M1 Garand semiautomatic medium-range rifle. Wherever possible they used the Thompson M1928 submachine gun, firing .45-caliber pistol ammunition, but this weapon was in short supply. Americans made do with their M1s, Browning Automatic Rifles (BARs), and light machine guns. It was late 1944 before they introduced the M3 submachine gun (grease gun) in large numbers to compete with the Schmeisser.
The Germans learned to exploit the weaknesses of Americans under fire for the first time. In such cases Americans had the tendency to freeze or to seek the nearest protection. All too often American infantry merely located and fixed the enemy, and called on artillery to destroy the defenders. Only after much experience in 1943 did American infantry learn that the best way to avoid losses was to keep moving forward and to close in rapidly on the enemy.
Tanks could not be used in the mountainous terrain of Italy in massed attacks as Rommel had done in Africa. In Italy tanks largely reverted to the infantry-support role that the British had envisioned for their Matildas and other “I” tanks at the start of the war. However, American tankers and infantry had little training in this role. Infantry and tanks could not communicate with each other. Infantry could not warn tankers of antitank traps and heavy weapons, and tankers could not alert infantry to enemy positions. Consequently, infantry had a tendency to lag behind tanks, and Americans did not work out the smooth coordination of tanks, infantry, and artillery that the Germans had developed long before in their battle groups or Kampfgruppen.
Similar problems developed in the use of tank destroyers (TDs), essentially 75-millimeter guns on open-topped tank chassis. TDs were designed to break up massed German panzer attacks. The Germans no longer massed tanks, but used them as parts of Kampfgruppen. American commanders slowly changed the use of TDs to assault guns to destroy enemy tanks and defensive positions with direct fire.
Finally, the Allies did a poor job of coordinating air-ground operations. Allied fighter-bomber pilots flying at 200 mph often could not distinguish between friendly and enemy forces on the ground. The pilots could not talk to ground units, and vice versa. This resulted in many cases of Allied aircraft bombing and strafing friendly forces. Consequently, Allied troops often fired on anything that moved in the sky. Only in the spring of 1944 did the U.S. Army Air Force deploy forward air controllers (FACs), using light single-engine liaison aircraft (L-5s) that could direct radio communication to aircraft and air-ground support parties at headquarters of major ground units. It was a bit late: the Germans had employed this system in the campaign in the west in 1940 to direct Stuka attacks on enemy positions.