Officially, the army has ignored counterinsurgency since the early 1970s, when the last American troops got out of Vietnam. Although the army has a long and successful history in fighting against irregular troops, Vietnam was seen as the wrong war at the wrong time. Back then, the army was focused on stopping the Red (Russian) Army from overrunning western Europe. Operations in Vietnam detracted from the main mission in Europe. Besides, guerilla wars were seen as old fashioned and not worth the bother for the modern, high tech force the army had become. After World War II, the army got blitzkrieg fever and became even less interested in the messy business of counterinsurgency. Army interest in counterinsurgency was low even before World War II.
Although army counterinsurgency operation in the Philippines a century ago were successful, the army entered the 20th century determined to get away from that sort of thing. So for the next four decades, the marines did most of the counterinsurgency work. The marines collected their experiences and published The Small Wars Manual in 1940. They revived its use during Vietnam. But the army, which was running things in Vietnam, overruled the marines. The Small Wars Manual did not disappear. The marines still kept it in stock, as did the U.S. Army Special Forces. In fact, the Special Forces had, since the late 1940s, had been developing good, solid, counterinsurgency doctrine. But the Special Forces had never fit in with the majority of army generals. It was only the intervention of President Kennedy in the early 1960s, and the need for the Special Forces during the Vietnam war, that kept the Special Forces, and their counterinsurgency expertise, alive. For the last thirty years, the Special Forces has been getting away from counterinsurgency, into things like commando ops, diplomacy and espionage.
But the army never got that far away from counterinsurgency. The Balkans peacekeeping operations of the 1990s found the army troops, if not the senior leadership, quick to relearn past lessons about counterinsurgency. Same thing happened in Afghanistan in 2001, and Iraq in 2003. Much that is in the new manual is old news to those familiar with U.S. Army history. The new manual (officially called F.M.I. 3-07.22) contains much that the marines put into their Small Wars Manual, and is also full of hard won wisdom from recent Iraq operations. The army has been very successful at counterinsurgency operations in Afghanistan and Iraq, and the war on terror. Now that success is recognized as part of the soldiers job, not just some exceptional emergency that had to be taken care of, then forgotten.
In record breaking time (five months), the U.S. Army came up with a new manual describing how troops should conduct counterinsurgency warfare. The manual itself is not as important as the fact that the army now, once again, recognizes counterinsurgency as a proper mission for troops.