The key events of the disaster took place over about three seconds.
A leak of high-temperature gas in the right solid rocket booster damaged the external stack of solid rocket boosters and external fuel tank.
The damage to the external tank caused it to come apart, and resulted in a fireball from the released liquid oxygen and hydrogen. The solid rocket boosters broke free of their mountings as the disaster unfolded.
The Challenger orbiter momentarily survived both the fireball and external tank failure. But those events made it impossible to keep the orbiter correctly oriented.
The loss of proper orientation while travelling ~1000mph meant Challenger was hit with very high stress forces, far beyond anything it was designed to survive, and swiftly broke apart.
Shortly afterward, the Range Safety Officer remotely detonated both solid rocket boosters, which had survived the disaster mostly intact and were careening out of control.
More detail at Wikipedia: https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disas...