The domestic situation in the US meant that successive Presidents had their hands tied and could not dedicate the necessary resources to the war effort.
The Northern Vietnamese communists employed highly effective military strategies to defeat the US and South Vietnamese.
The communists didn't win the war, the US lost it due to extensive flaws within its war strategy.
An economic crisis severely limited the financial resources at Johnson's disposal.
In 1968, the Western world was rocked by the most significant economic crisis since the Great Depression. The crisis was a watershed moment for most western economies; it signalled the end of the post-Second World War economic boom.
The economic crisis began in March of 1968. A “chronic” US balance-of-payments deficit, exacerbated by an increase in imports from Europe, and a surge in US tourists spending their money overseas caused an increase in dollars heading abroad. This weakened confidence in the dollar, causing inflation. Foreigners, panicked by the weakening dollar, sold dollars and bought gold, causing the “largest goldrush in history”. Johnson had attempted to preempt the economic crisis by proposing a tax increase in 1967. But the legislature had been held up in Congress by conservatives who preferred that Johnson cut domestic spending on his “Great-Society” reforms instead. In early 1968, Congress and Johnson reached a compromise. Johnson would cut domestic spending by $6 billion, and Congress agreed his tax increase, but it was too late to avoid the worst impacts of the crisis. Johnson himself cited the financial crisis of 1968 as a major reason for not escalating the Vietnam war. He said, “budgetary problems were constantly before us as we considered whether we should or could do more in Vietnam”. He knew that increased military spending in Vietnam would put more pressure on the dollar and could lead to further weakening. The dollar simply couldn’t support it.
If Johnson had really wanted to escalate the Vietnam War, he could have made greater spending cuts domestically. He tried to fund the Vietnam War, and his ambition “Great-Society” welfare reforms simultaneously, meaning neither got the budget required to make the policies a success.  Had he have made the Vietnam War his sole priority, he would have had significantly more financial resources available. Also, Congress permitted exceptions in cases when it restricted funding. For example, the McGovern-Hatfield amendment of 1970 only prohibited funds for certain military activities, including escalating troops. The President had plenty of political space to pursue other military avenues aside from escalating troop numbers.
The economic slowdown of 1968 caused Lyndon B. Johnson to cap the escalation of the Vietnam War. He could not longer afford to finance a constantly-escalating war half-way across the globe.
Enter the technical rejections of the premises here ...
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