While the Lebanese civil war is commonly treated as an internal sectarian conflict, A Third World War instead reconstructs the worldviews of the protagonists to show how the conflict was an important setting in the international civil war that defined the twentieth century. It traces the emergence of a revolutionary, internationalist, anti-sectarian subjectivity that united Lebanese citizens and Palestinian refugees into a coalition against what they called “imperialism, Zionism, and Arab reaction.” This prompted their opponents to mobilize their own counterrevolution anchored by visions of sectarian and national purity to combat what they referred to as an “international leftist conspiracy.” Meanwhile, US policy placed significant emphasis on maintaining Lebanon’s pro-Western orientation, laissez-faire economy, and sectarian political system at a time when much of the Arab world appeared to be turning in the opposite direction. The competition between these rival networks erupted into civil and international war in 1975 and continued until the end of the Cold War. The war’s decisive initial phase ended in 1977 with the defeat of the Lebanese revolutionary bid, the division of the country between Syrian and Israeli areas of influence, and the assassination of Kamal Junblat, the lynchpin of the alliance between the Lebanese National Movement and the Palestine Liberation Organization. This conjuncture prompted a profound disillusionment in the Arab east while clearing the ground for the 1978 Camp David accords. Drawing on newly declassified or long-neglected American, Lebanese, Palestinian, and British sources in Arabic, English, and French, I explore how Middle Eastern actors employed and transformed transnational networks and discourses of conflict, while arguing the Lebanese civil war was a crucial setting for the US and its allies after Vietnam. Using what I call the “Third World War” as a conceptual framework, I track the linkages between people, materiel, and ideas across sites of conflict, contributing to the rethinking of the Cold War as an international civil war connecting actors in numerous states, and often driven by events in the so-called periphery.