Examples of famous problems resolved easily Have there been examples of seemingly long standing hard problems, answered quite easily possibly with tools existing at the time the problems were made? More modern examples would be nice. An example could be Hilbert's basis theorem. Another could be Dwork's p-adic technique for rationality of zeta functions from algebraic varieties.

The Dinitz conjecture maybe doesn't belong here because it wasn't all that famous, but it was an open problem for over ten years and the proof was really, really trivial.
