The conclusion of the US Army's 1954 pamphlet, German Antiguerilla Operations in the Balkans (1941-1944) (German Report Series No. 23-243), did actually conclude that German brutality was in no small measure to blame for the failure of German efforts in the Balkans. Based largely upon German sources and supplemented with Allied observer reports, the pamphlet's authors critiqued the the Axis's divided command structure, manpower shortages, arrogance, and rapacity as one of the major reasons for the failure to destroy the guerillas (the authors also engage in a bit of demi-orientalism, describing the Balkan peoples as "hardy Greeks and Slavs, traditionally opposed to any occupier"). The pamphlet noted that German policy reduced the civilian population to subsistence levels and thus made it much harder to raise a cadre of native collaborators. Somewhat surprisingly given the hostage policy , the study also concluded that German policy towards the "the outwardly cooperative portion of the population was far more lenient than security demanded," as German overstretched German troops had to rely upon the civilian population as a labor force, allowing the guerrillas to penetrate further into the occupation regime. The pamphlet ends with an encapsulation of how the lessons of German failure can help develop better policies in the future:

However, a review of the mistakes these commanders made would undoubtedly cause them to urge any future occupier [of the Balkans] to begin his administration with a clear-cut statement of policy, including a promise of eventual withdrawal of occupation troops and self-determination for the people; a unified military command and distinct delineation of responsibility in the political and military fields; the assignment of trained, well-equipped combat troops in adequate numbers to the area; the taking of prompt and effective though not excessively harsh measures to quell disorders; and an extensive propaganda campaign to explain the purpose of the occupation and the benefits to accrue to the population with the maintenance of law and order. Finally, they would most certainly recommend the troops be supplied from outside the country and restrained from excesses. With perseverance, the occupation forces might then be able to avoid the Balkan chaos of 1941-44.

Whether or not this pamphlet, or any other historical study of German operations, had any influence on US Army COIN doctrine is more difficult to say, but it is notable that the authors of the pamphlet used the German Balkan experience to formulate something similiar to "hearts and minds."

The rise of modern counterinsurgency goes hand in hand with the rise of modern guerrilla warfare. The development of counterinsurgency during the Cold War (leading up to today) can not be explained without understanding what made guerrilla warfare 'modern' in the context of the late 1940s (and forward). Let's begin there.

The origins of modern guerrilla warfare can be found in Mao Zedong and the Chinese Civil War. Beyond creating an actual theory for which you could base how to fight a guerrilla war on, Mao Zedong's victory in the Chinese Civil War and those that came afterwards signified the fact that guerrilla warfare had now become politicized and driven by ideology. No longer was it only about focusing on military matters, of fighting your enemies - now you had to gain people's support for your cause. It meant that guerrilla fighters had to be politically aware and that their leaders had to adapt to the 20th century. Because of this, propaganda became a vital component of guerrilla warfare and the concept of engaging an international audience in your cause was developed through the use of printed material to the radio and finally to television and the Internet in the modern age. The Viet Minh during the French Indochina War, for example, added the element of globalization through the concept of engaging international opinion - something that was made possible by technological advances of the 1950s.

Traditionally, those fighting the insurgents had always tried to separate the people from the insurgents but now the concept of doing so became more urgent and more difficult than ever. Insurgencies became to focus more and more on being able to control and influence the people. The Viet Minh (again) even went so far as to carry out a highly successful literacy campaign in Vietnam which came to serve as a way to also gain the support of the people while at the same time making the people available for indoctrination.

For those fighting insurgents using these sophisticated strategies, you had to use equally sophisticated strategy to counter their efforts. You simply could not defeat the insurgents without also defeating their ideas. You couldn't simply discredit their ideas without providing them with a better alternative or getting at the root cause of the insurgency. While separating the population from the insurgents is a classic counterinsurgency strategy, it took a new urgency. Modern counterinsurgency began to move away from a purely military endeavour to a far more intricate, more civilian driven cause in which the military means of waging war was subordinate to a civilian socio-economic strategy.

I would like to take a moment to say that I am speaking in general terms at the moment. This does not reflect fully on a very complex reality in very complex conflicts. Yet it is at the very core of modern counterinsurgency and modern guerrilla warfare that this is a war waged over the people; conflicts in which the one side doesn't focus on the people usually end with the other side winning (see, for example, Che Guevara in Bolivia as a guerrilla example and the Soviet Union in Afghanistan as a counterinsurgency example). The French professional soldiers that laid out the modern French counterinsurgency theories before the Algerian War were so taken by the politicization by the insurgents of the First Indochina War that it radicalised the French officer corps and produced such absurdities as Col. Charles Lacheroy's guerre revolutionnaire which posed that there was a worldwide Marxist conspiracy and that all insurgents in the world were secretly or openly communists - something which was actually an accepted concept by the French professionals entering the Algerian War!

As /u/kieslowskifan points out, the United States were already aware that brutality would not win a fight against insurgents, something they themselves were well aware of since the Philippine-American War 40 years earlier. While the pamphlet suggests many important lessons, it fails to obviously point out the fact that Germany was so bound by its ideology to make such a change improbable in the historical context.

To start summing this up, the development of COIN after WWII comes off as a reaction to an entirely new world. The road to counterinsurgency as we know it today was difficult, dirty and heavy; even Great Britain, fighting a successful and humane counterinsurgency conflict in Malaya during the 1950s found itself fighting a more brutal and less successful conflict in Kenya during the same decade. It would fail in Aden during the 1960s but succeed in Oman ten years later. There was no set military doctrine and the doctrines that grew out of the early counterinsurgency conflicts of the Cold War came to continuously change and develop over the decades, but most importantly, they were all forced to adapt to local conditions. Like I've mentioned to you before in a previous thread, there is no 'one size fits all' doctrine. There are some aspects and elements you could focus on and you could certainly take in lessons learned from previous conflicts (such as not being brutal towards civilians, as the Nazi example shows us) but it was vital to understand and consider where you were fighting, what you were fighting for and who you were fighting against.