Returning to the issue of political violence, a two-generation cycle is also evident in the United States's history. There were bursts of political violence around 1870, 1920, and 1970, though not around 1820 -- people may have felt too good for that back then.
This suggests that the US is due for another outbreak of political violence around 2020, and we may already be seeing some of that, like the Proud Boys. Combined with being in a disintegrative phase, this could be very big trouble.
Turning to foreign policy,
US History Cycles - Extroverted vs. Introverted foreign policy describes Frank Klingberg's work on foreign-policy cycles, drawing from politicians' speeches, naval budgets, and territorial acquisitions. As with the other cycles, each phase is self-limiting. Introverted phases end as a result of unmet challenges from other nations, whether real or perceived, while extroverted phases end as a result of burnout from big wars.
The US started out as introverted, because fighting for independence was a lot for it, I'm sure. Then an undeclared naval war with France provoked an extroverted period that ended with the War of 1812. Then two decades of introversion that involved confrontation with South Carolina of its would-be nullification of some tariffs, whether to admit Texas being a big issue, and being unwilling to help the Canadian rebels in the early 1830's. This was followed by an extroverted period with annexations of Texas and the Oregon Territory, the Mexican War, and then the Civil War. Then an introverted period that involved passing on the Europeans' Scramble for Africa. Then an extroverted period that started with fighting Spain in Cuba and the Philippines, and that ended in World War I. The resulting introversion was strong enough to keep the US out of the League of Nations. But the rise of the Axis powers doomed it, though it has a last gasp in the form of the America First movement. On December 7, 1941, Japan ended it by attacking Pearl Harbor, and the US became extroverted again. Every present member but one of the House voted to declare war on Japan, as did every present member of the Senate.
This extroversion lasted through the early decades of the Cold War, with the US being involved in Korea and Vietnam. According to Peter Klingberg and some others, this extroversion ended around 1968 or so, with the US turning the fighting of the Vietnam War over to South Vietnam, and also improving relations with the Soviet Union -- the détente era. Jimmy Carter continued the trend by issuing a "pardon" to draft dodgers, something that he distinguished from an "amnesty". He also turned the Panama Canal back over to Panama. Reagan seemed very extroverted about foreign policy, with his seemingly wanting some big confrontation with the "evil empire", but he and others bellyached about "Vietnam syndrome" -- introversion. The US became more extroverted in the late 1980's, with its participation in the Gulf War in 1991, in Afghanistan in 2001, and in Iraq in 2003, and more generally, in the "War on Terror".
But the US has been winding down those wars, with the US pulling out of Iraq in 2011. Its involvement with Libya's civil war in 2011 was mainly as an air force without a lot of troops on the ground. The Obama Administration has been a big user of drones, and the Trump Administration an even bigger user, likely because drones are a low-investment sort of weapon -- much cheaper than sending some big expeditionary force like in 1991.
So the US has likely entered an introverted phase of foreign policy, one without anything much bigger than drones.
