Why did Challenger break apart?

The hardware failure

The familiar word 'exploded' hides the mechanism. The Commission traced the accident to failure of the pressure seal in the aft field joint of the right solid rocket motor. Its account concerns the joint, escaping gases and breakup of the vehicle.¹

The cold mattered to a seal already carrying a history of problems. A successful earlier flight did not establish safety in the next flight's conditions.¹²

Why the launch went ahead

The Commission found that decision-makers lacked the full history of O-ring problems and did not know of continuing engineering opposition after contractor management reversed its recommendation. It described communication failures and conflict between engineering data and management judgment.²

Chapter 5 is about matching the tool to the job. Is the room assessing safety or looking for permission to proceed? Those are different tasks.

Why did Challenger launch despite warnings?

Engineering objections did not reach the final decision in a complete form. The inquiry says that, had the decision-makers known all the facts, it was highly unlikely they would have launched that day.²