Gordian knot

noun2

1. •
   The legendary knot tied to a pole near the temple of Zeus in Gordium.
2. •
   Any intricate and complex problem having a simple solution.

   • List his discourse of war, and you shall hear A fearful battle rend'red you in music; Turn him to any cause of policy, The Gordian knot of it he will unloose. — 1599