The U.S. Army is rediscovering lessons they learned, and ignored, over half a century ago. Now it's called hybrid warfare. Back then it was called using everything to beat the enemy, and that included dealing with guerillas working for, and against, you. It meant coping with lots of civilians on the battlefield, and coping with the media. Hybrid warfare is also the realization that irregular warfare, the media and civil affairs has also been changed by advances in technology.
The U.S. went into World War II realizing that guerilla warfare would be an ally. In the Philippines and Europe, local guerillas were a major asset for American forces. In both cases, the United States had been working with the guerilla groups for over a year before American troops showed up. During World War II, the U.S. controlled domestic media and engaged in media wars against enemies and allies.
But after war, friendly, or hostile, guerillas were largely forgotten, along with "Information War" (as it was later called) . The main problem with guerilla warfare is that the American armed forces take it for granted. U.S. troops have been defeating guerilla movements for centuries. Through all that time, guerilla warfare has been the most frequent form of warfare for American troops. But guerilla warfare has always been viewed, by the brass, as a minor, secondary, military role. It never got any respect. Even the U.S. Marine Corps, after half a century of guerilla warfare operations, were glad to put that behind them in the late 1930s. All that remained of that experience was a classic book, "The Small Wars Manual," written by some marine officers on the eve of World War II. That book, which is still in print, contained timeless wisdom and techniques on how to deal with guerilla warfare operations, and "small wars" in general. Much of the work the army has done in the last five years, to revise their manuals, could have been done just by consulting the Small Wars Manual. In some cases, that's exactly what was done.
The basic truth is that the fundamental principles of guerilla warfare have not changed for thousands of years. What has also not changed is the professional soldier's disdain for guerilla warfare operations. This sort of thing has never been considered "real soldiering." But the U.S. Army and Marines have finally come to accept that guerilla warfare is a major job, something that U.S. troops have always been good at, and something that you have to pay attention to. So when you see more news stories about the "new American guerilla warfare manual", keep in mind the history of that kind of warfare, and how long, and successfully, Americans have been doing it.
Now we have a lively discussion about "hybrid warfare", where conventional forces learn to operate with irregular fighters, just like they did during World War II, and as long ago as the American Revolution. But by making a big deal out of Hybrid Warfare, the army is committing itself to remembering what it is, and training troops to handle it, in addition to conventional operations.
Hybrid warfare also includes Cyber War, a form of conflict that has also shown up frequently in the past, but not with the extensive technology available today (particularly the Internet). Media based warfare is also more of a factor, again because of technology.