The official story on the Gulf of Tonkin Incident was later   shown to be a pack of lies, as a pretext for escalating the Vietnam War.

The true story of the Israeli attack on the USS Liberty is still murky.
A BBC documentary suggests the Israeli attack would be blamed on the Egyptians to give the US an excuse to attack Egypt. The true story is still hidden in layers of conflicting narrative, and the fact no clear unambiguous explanation has emerged suggests collusion and hankypanky still holds sway.

The MyLai massacre was covered up for 18months before hitting headlines. 3 US servicement who tried to protect civilians were shunned, and even denounced as traitors by several U.S. Congressmen, including Mendel Rivers, Chairman of the House Armed Services Committee. Only after thirty years were they recognized and decorated, one posthumously, by the U.S. Army for shielding non-combatants from harm in a war zone.

Truth is an elusive beast and takes time to surface, sometimes it never does!
Just three examples.There are many others. Each is a can of worms.
Surely no one ever takes anything purely on trust anymore?