1971 Hanoi and Red River Delta Flood
Deemed one of the "most serious weather events of the century" (in fact, it was one of the deadliest weather events in human history), the Red River Delta flood of 1971, may have claimed as many as 100,000 lives. The exact number of lost lives could have been much higher, however. It's difficult to know for sure because the event happened in the middle of the Vietnam War, so details about the flood are limited.
The area of flooding in and around Hanoi is highly populated, so there is little doubt that the destruction was immense. The fact that the event happened during wartime only heightened the disaster.