Stranger, you are quite correct to note the earlier invasions by Chinese and Mongols.
No story in history is ever simple. Johnson did not understand that he was wandering into a buzz saw. Being a good Texas boy, he couldn’t walk away from a fight … much to his (and our) cost.
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Here’s an op-ed piece in the Washington Post about LBJ’s accomplishments while in office: linky.
This article is very positive about LBJ. No surprise there - it’s by Joseph Califano, former member of LBJ’s White House staff. It’s worth noting anyway - LBJ did get a lot done while in the White House. Historians will wrestle with the balance of good and bad in LBJ’s administration for a long time to come. Historically inclined Dopers as well.
Anyone interested in reading further into this subject can go to Wowio. For those unfamiliar with the site, it lets you download free books as PDF files. Some of the ones currently available are Death of a Generation: How the Assassinations of Diem and JFK Prolonged the Vietnam War by Howard Jones and Into the Quagmire: Lyndon Johnson and the Escalation of the Vietnam War by Brian VanDeMark. Those looking for a short overview might be interested in How Did It Happen? The Vietnam War by Clive Gifford.