Banana Wars, while fighting bandits, freedom fighters and insurgents in places like Haiti, the Dominican Republic and Nicaragua that United States Marine Corps aviators began to experiment with air-ground tactics to support Marines on the ground. In Haiti the Marines began to experiment with dive bombing and in Nicaragua where they began to perfect it. While other nations and services had tried variations of this technique, Marine aviators were the first to include it in their tactical doctrine. Germany was banned from possessing an air force by the terms of the World War I armistice. The German military continued to train its soldiers as pilots clandestinely until Hitler was ready to openly defy the ban. This was done by forming the Deutscher Luftsportverband, a flying enthusiast's club, and training pilots as civilians, and some German pilots were even sent to the Soviet Union for secret training; a trained air force was thus ready as soon as the word was given. This was the beginning of the Luftwaffe. World War II Military aviation came into its own during the Second World War. The increased performance, range, and payload of contemporary aircraft meant that air power could move beyond the novelty applications of World War I, becoming a central striking force for all the combatant nations. Over the course of the war, several distinct roles emerged for the application of air power. Strategic bombing Strategic bombing of civilian targets from the air was first proposed by the Italian theorist General Giulio Douhet. In his book The Command of the Air (1921), Douhet argued future military leaders could avoid falling into bloody World War I–style trench stalemates by using aviation to strike past the enemy's forces directly at their vulnerable civilian populations. Douhet believed such strikes would cause these populations to force their governments to surrender. Douhet's ideas were paralleled by other military theorists who emerged from World War I, including Sir Hugh Trenchard in Britain. In the interwar period, Britain and the United States became the most enthusiastic supporters of the strategic bombing theory, with each nation building specialized heavy bombers specifically for this task. Japanese strategic bombing Strategic bombing, mostly targeting large Chinese cities, was independently conducted during the Second Sino-Japanese war and World War II by the Imperial Japanese Navy Air Service and the Imperial Japanese Army Air Service. There were also air raids on Philippines and Australia, as well