War broke out between the French and Ho Chi Minh primarily due to the failure of diplomatic discussions and the inability to reach a consensus on Vietnam's future after World War II. The Geneva Accords, which aimed to resolve the conflict and establish a peaceful framework for Vietnam's independence, ultimately proved unsuccessful. The French sought to maintain their colonial control, while Ho Chi Minh and his supporters were determined to gain full independence for Vietnam. Thus, the answer is:
3. Diplomatic discussions with the French were unsuccessful.
While the Geneva Accords were indeed a significant factor, the prime triggering event for the escalation to war was the breakdown of diplomatic negotiations.