Four-color theorem (1976)
Kenneth Appel and Wolfgang Haken proved that four colors suffice to color any map, relying on 1,936 cases checked by computer. The proof ran to hundreds of pages, and no human checked every case by hand.
Mathematicians accepted the result but debated whether a computer-assisted proof counted as a proof.
It opened the field of computer-assisted mathematics and set a precedent for trusting machine-checked cases.
The Yang-Mills proof is far larger, and its acceptance depends on software that checks every step rather than on humans reading it.
