The upheavals in the USSR and the resultant Third World War from 1971-1975 ranks among the bloodiest conflicts in human history. I think that it need not have been so, however. There are various points at which great bloodshed could have been avoided. A common example cited is stopping the assassination of Brezhnev and thus the Soviet Civil War initiated by the Red Army brass, which caused the instability out of which nationalist uprisings grew like wildfire.
Even after that, however, the war still could have been contained within the USSR if Ceaușescu hadn't sent forces into Moldavia and precipitated the Warsaw Pact's attempted joint intervention, which of course resulted the war spilling over into the west with the Third Balkan War between Albania, Yugoslavia, Greece, Turkey, and Bulgaria, and the resultant NATO intervention, which created the conditions under which war was sparked between the Germanies.
Even this still could have remained a general European war if it hadn't been for China's intervention in the Primorye and Central Asia, which resulted in US involvement in the east and Westmoreland's encirclement plan, along with it the invasions of North Korea, the opening of a massive northern offensive in Vietnam and Laos, and the cooperation with India in its invasion of Tibet (which fell apart after the beginning of war with Pakistan).
The invasion of China resulted in the majority of the war's casualties, from North Korea's tooth-and-nail resistance against the SK/US/Japanese offensive (and the human rights abuses of Japanese troops, especially the looting and rape in Wonsan), to the destruction of the river dams in North Vietnam and the subsequent deaths of millions of Vietnamese due to the flooding and famine which followed, to the total war in India which saw the first use of Nuclear weapons in warfare since 1945.
Beyond the Pacific, chemical and biological warfare in Central Asia decimated the region's productivity and population, the Chaco War between Peru, Bolivia, Venezuela, and Paraguay against Brazil, Argentina, Chile, Ecuador, and various factions in Colombia resulted in yet more casualties, the Mexican Civil War between the state and neo-Zapatista rebels in Yucatan, Chiapas, Oaxaca, and Guerrero nearly brought war to US soil, the war in Europe destroyed so much of the development of the last 30 years and resulted in decades of economic and demographic downturn and cultural trauma, and the upheavals in the US resulted in the top blowing off of many of the CIA's domestic operations, and the opening of those records let the light of day shine on the extent of US crimes.
By the end of the war in 1975, an estimated 90 million people had died, among them 7 million US soldiers, the most by far ever lost in a single conflict.
All of it, however, could have been avoided. The USSR might have maintained its integrity and the Civil War might never have occurred, or the USSR might have been negotiated out of existence, and its constituent states granted independence, or the conflict might not have spread so far. So many possibilities to avoid war.