Why is it difficult to say when the Vietnam War began?

Explanation:

The U.S involvement in Vietnam is hard to create a concrete pinpointed date for the beginning of the war because the growing tension between Communist Vietnam and America had been accumulating over a number of years. ... The so-called "domino theory" dominated U.S. thinking about Vietnam for the next decade.

The Vietnamese terrain and climate were harsh and difficult. U.S. soldiers were poorly equipped and lacked adequate training.

The Vietnam War and active U.S. involvement in the war began in 1954, though ongoing conflict in the region had stretched back several decades.

According to the New York Times, in 1996 Congress recognized the start of U.S. involvement in the Vietnam War as Feb. 28, 1961, when U.S. military advisers began accompanying South Vietnamese troops on operations. The war's start was formerly established as Aug. 5, 1964, when President Lyndon B.