To what extent was the Vietnam War the major turning point in US foreign policy between 1898 and 1991?

Between 1898 and 1991 American foreign policy encountered three major turning points: The moveto imperialism and global expansion beginning in 1898; the inward turn to isolationism following thefinancial crash of 1929; and the Korean Cold War engagement of 1950 that mapped out aninternationalist ideological position for foreign policy that remains in place today. During theseperiods, there are moments of considerable significance which may appear as turning points,conversely it can be argued that foreign policy remains essentially unchanged in the subsequentperiod. The Vietnam War and the Truman Doctrine both fall into this category.
In 1898, America was on the verge of entering the global stage as a great power. Before the year wasover, the annexation of Hawaii and swift military victories over Spain in the Philippines, Puerto Ricoand Cuba had delivered sugar, mineral wealth, new markets for American goods and a secure Pacificnaval base in Manila to combat any European or Japanese threat to the China Open Door Policy. Withthe successful open-door note negotiations between 1899 and 1900, the McKinley administration hadguaranteed open access to the Chinese market for the growing industrial dominance of the US. In thefive years up to 1900, exports to China had quadrupled to $15 million. It was a remarkable transitioninto imperialism, while adding only 125,000 square miles of territory. The interest was not land, butmarkets. The period represents a vital turning point in American foreign policy, one quickly followedby another significant global move.