Which event most contributed to the changing troop levels shown in this graph?
the twenty-sixth amendment lowered the draft age to 18 from 21.
u. s. and north vietnamese ships exchanged fire in the gulf of tonkin.
congress expanded presidential powers to wage war under the war powers act.
communist troops launched a series of attacks during the tet offensive.

U.S. and North Vietnamese ships exchanged fire in the Gulf of Tonkin.

The answer is:

U.S. and North Vietnamese ships exchanged fire in the Gulf of Tonkin.

The Gulf of Tonkin was used by different American ships to spy on North Vietnamese radar towers, on July the submarine encountered three Soviet-made Vietnamese submarines and opened fire, this started the war on Vietnam as several shots were fired and after that the president passed the Tonin Gulf Resolution that gave him the power to declare war in southasia as he saw fit, after this more and more troops were deployed in Vietnam.

the monroe doctrine is the best known u.s. policy toward the western hemisphere. buried in a routine annual message delivered to congress by president james monroe in december 1823, the doctrine warns european nations that the united states would not tolerate further colonization or puppet monarchs.

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