 At the dawn of the 20th century, the United States found itself at war with first Spain and then the Philippines. These foreign interventions set the tone for future American foreign policy. Writing around this time, Mark Twain gave an ominous warning about foreign policy by a great republic. But it was impossible to save the great republic. She was rotten to the heart. Lust of conquest had long ago done its work. Trampling upon the helpless abroad had taught her, by a natural process, to endure with apathy the like at home. Multitudes who had applauded the crushing of other people's liberties lived to suffer for their mistake in their own persons. The natural process Twain describes occurs through a framework we call the boomerang effect. In the absence of strong formal constraints, tactics used in foreign interventions abroad are later used to limit the liberties of people back home. Like a boomerang, the U.S. government aims its policies abroad, only for them to return back to the United States. This occurs through the development of human capital, physical capital, and an interventionist mindset which are necessary to control others. One illustration of the boomerang effect is the militarization of domestic policing. Historically, the American police and military served two distinct functions, the police protected and served the American public, while the military defeated external threats. Over time, that distinction was blurred. In 1967, for example, ongoing race riots left the LAPD in search of more effective crowd control methods. Former Marine John Nelson provided a solution. During his deployment in Vietnam, Nelson served on a special force recon unit. Nelson had these accurate and aggressive tactical units in mind when he proposed creating SWAT teams to aid the LAPD in crowd control. Whereas the average Marine unit would kill about 7.6 people per man lost, a force recon unit would kill about 34. And whereas a regular Marine unit would initiate combat with the enemy, only about 20% of the time, a force recon unit would initiate combat 95% of the time. This demonstrates the role of human capital. Intervening abroad provides a learning environment in which participants obtain a unique set of skills to coercively control other people. The skills developed abroad return home as experiences and training from foreign intervention are transferred to domestic positions. Implementing Nelson's idea required somebody with some rank and some hutzpah to champion it. Daryl Gates, a World War II veteran and LAPD inspector, filled this role. He understood and approved of Nelson's proposal and spearheaded the development of the first SWAT team. SWAT teams became more widespread at the end of the Cold War as policymakers focused on initiatives like the War on Drugs. The physical capital of the military, such as tactical gear, assault rifles and armored vehicles, was donated to domestic police throughout the United States by the federal government. This allowed even small agencies to create SWAT units. This combination of human and physical capital contributes to more aggressive forms of domestic policing. An example is the no-knock raid, where police officers enter the personal property of citizens without notifying them, without knocking. The number of no-knock raids has risen dramatically over time. Since 9-11, a series of federal programs have made state and local police increasingly reliant on the federal government for training, equipment and intelligence, as well as subject to the federal government's rules, regulations and influence. This process of centralization places more power in the hands of the federal government and weakens the checks created by dispersed political power. Domestic police, who once focused on protecting citizens, begin to view citizens as potential enemies who must be monitored and controlled, threatening their civil liberties. To learn more about the boomerang effect and its impact on domestic civil liberties, buy our new book, Tyranny Comes Home, The Domestic Fate of U.S. Militarism.