Which mission marked the beginning of the Apollo program?

The mission that marked the beginning of the Apollo program is Apollo 1. This mission, originally designated AS-204, was intended to be the first crewed flight of the Apollo program. Although it did not result in a successful flight due to a tragic cabin fire during a pre-launch test, Apollo 1 was significant in establishing critical safety measures and protocols that would influence the design and operation of subsequent missions.

Apollo 1 was intended to demonstrate the spacecraft's systems in a real-world operational environment and lay the groundwork for future missions. The lessons learned from the Apollo 1 incident led to numerous safety enhancements and engineering changes within the Apollo program, proving essential for the successful missions that followed, including Apollo 7, which was the first successful crewed mission, and the iconic Apollo 11, which achieved the first lunar landing.