Revolutionary War
The Revolutionary War resulted in the birth of the United States and had a hugely important impact on King's College, which would become Columbia (including, you may have guessed by now, that very name change).
During the conflict, the college was closed. The King's building, College Hall, was a revolutionary barracks and then a British military hospital for a time. Even the future grounds of the Morningside Heights campus saw action during the Battle of Harlem Heights, an event commemorated by a plaque on Mathematics Hall.
King's alumni and affiliates fought in the war as well, including Alexander Hamilton.