Although the Tokugawa regime was declining for many decades and faced many problems, what was the event that sparked the crisis and led to the downfall of the regime

The arrival of Americans and Europeans.

Explanation:

The arrival of Americans and Europeans in the 1850s increased internal tensions that led to the downfall of the Tokugawa regime. The old political structure and weak economy against the Europeans and Americans also contributed in the downfall. Western powers aim on opening Japan to trade and foreign intercourse and due to weak economy, the government was unable to stop this interference of foreign countries that leads to downfall.