Why did US leave the Vietnam war after it has almost won ?

The simple but inconvenient answer is: Israel wanted to funnel all US military aids to itself and made US withdraw from all other fronts.

The following details have been found from historical records:
1/- The USS Liberty was attacked by Israel (false flag operation) to draw US into war against Arab States. Fortunately the crew survived to tell their stories. The Soviet Navy happened to be nearby and was witnessing. US President Johnson banned crew members from speaking out.
2a/_ a Feb. 1, 1972, meeting between Nixon and evangelist Billy Graham in which “there was considerable discussion of the terrible problem arising from the total Jewish domination of the media and agreement that this was something that would have to be dealt with.” (Haldeman’s Diaries Show Nixon’s Dark, Human Sides : History: Secret memoir tells of President’s alternate glee and guilt at provoking antiwar demonstrators.).
2b/- “Rev. Billy Graham openly voiced a belief that Jews control the American media, calling it a “stranglehold” during a 1972 conversation with President Richard Nixon, according to a tape of the Oval Office meeting released Thursday by the National Archives.
“This stranglehold has got to be broken or the country’s going down the drain,” the nation’s best-known preacher declared as he agreed with a stream of bigoted Nixon comments about Jews and their perceived influence in American life.
“You believe that?” Nixon says after the “stranglehold” comment.
“Yes, sir,” Graham says.
“Oh, boy,” replies Nixon. “So do I. I can’t ever say that but I believe it.”
“No, but if you get elected a second time, then we might be able to do something,” Graham replies.
Later, Graham mentions that he has friends in the media who are Jewish, saying they “swarm around me and are friendly to me.” But, he confides to Nixon, “They don’t know how I really feel about what they’re doing to this country.”
…Haldeman’s diaries noted the conversation. He wrote that there was discussion “of the terrible problem arising from the total Jewish domination of the media, and agreement that this was something that would have to be dealt with.”
He continues, “Graham has the strong feeling that the Bible says there are satanic Jews and there’s where our problem arises.”

…Nixon then broaches a subject about which “we can’t talk about it publicly,” namely Jewish influence in Hollywood and the media. He cites Paul Keyes, a political conservative who is executive producer of the NBC hit, “Rowan and Martin’s Laugh-In,” as telling him that “11 of the
12 writers are Jewish.”
“That right?” says Graham, prompting Nixon to claim that Life magazine, Newsweek, The New York Times, the Los Angeles Times, and others, are “totally dominated by the Jews.” He calls network TV anchors Howard K. Smith, David Brinkley and Walter Cronkite “front men who may not be of that persuasion,” but that their writers are “95 percent Jewish.”

The same thing happened with Soviet Jewry. Nixon and Kissinger may have opposed the community’s support for the Jackson-Vanick amendment linking trade tariffs to emigration, but they went along with it when they had to. Fisher, a lifelong Republican, now says (in Goldberg’s book) that Nixon may have been right, after all.
‘History has proven I was probably right when I opposed Jackson-Vanik,’ says Fisher. ‘Nixon was trying to work it out by personal diplomacy,’ Fisher told Goldberg. ‘Kissinger thought he could get out 30 or 40,000 a year if they’d worked that angle. When you think of it over 20 years, you could have gotten out quite a number of Jews.’
(Richard Nixon – Henry Kissinger
The Anti-Semite who saved Israel
By Jerry Klinger).
3/- In 1972, the An Lộc battle for 66 days (Battle of An Lộc – Wikipedia) was a victory for South Vietnam but was reported as a big loss in US fake news.
4/- Nixon strategy is “As for the third name on Haig’s list, former Defense Secretary Clark M. Clifford was not only a reminder of Nixon’s near-political-death experience with the 1968 Bombing Halt, but more recently of his biggest foreign policy secret, his “decent interval” exit strategy from Vietnam. Nixon’s announced strategy was “Vietnamization and negotiation,” his slogan for training the South Vietnamese military to defend its government itself and for securing the Communists’ agreement to abide by the results of elections in the South. The goal was a self-defending, self-governing South Vietnam, and Nixon stood firmly behind those goals in public, even as he abandoned them in private. By the time of the Pentagon Papers leak, he had settled on a “decent interval” strategy. Since Vietnamization was not going to achieve the goal of self-defense, Nixon secretly decided not to bring the troops home until shortly before or after the 1972 presidential election. And since he couldn’t force Hanoi to abandon military conquest of the South, he was trying to negotiate a “decent interval,” a period of a year or two between his final, election-timed troop withdrawal and Hanoi’s final military victory. This way he could arrange the appearance that his “Vietnamization and negotiation” strategy had succeeded, even though he expected it to fail. Publicly, he claimed great political courage for prolonging an unpopular war, but really it was political calculation. If he admitted that he added several years and several thousand casualties to the war and still lost it, he would go down in the 1972 election and in history as a President who lost a war. He was determined to avoid that, even if it cost American lives.” (A Rough Guide to Richard Nixon’s Conspiracy Theories | Miller Center).
5a/- Nixon ordered full military aids to Israel in the Yom Kippur War in 1973 (Yom Kippur War – Wikipedia) .
All that went by the boards with his first call to Kissinger on October 9, at 1:45 in the morning. Kissinger exhausted by the previous day, listened quietly as Dinitz explained that Israel was desperately short of supplies, had suffered heavy losses, and was near defeat. An emergency airlift was absolutely imperative.”6
“At the early-morning meeting, Dinitz and his military advisers briefed Kissinger that the Israeli losses were frightful, Forty-nine aircraft down, with fourteen Phantoms among them. But the figure that staggered Kissinger was the tank losses – almost five hundred destroyed in less than four days of fighting. No nation, not the United States, not even the Soviet Union, could make up such losses if the rate was sustained (Richard Nixon – Henry Kissinger The Anti-Semite who saved Israel By Jerry Klinger).
5b/- On the other hand, US failed to act when North Vietnam openly violate the Paris Peace Accords (Paris Peace Accords – Wikipedia) Henry Kissinger and Congress also tried to cut off military aids to South Vietnam, against President Ford’s intention, even during the final battles in Feb. 1975.
6/- Nixon feared that a “Jewish cabal” at the Bureau of Labor Statistics was skewing data to make him look bad, and he instructed his aide Fred Malek to tally up the Jewish employees at the bureau—a count that probably resulted in the demotion of two Jews. (It later forced Malek’s own resignation from George Bush’s 1988 presidential campaign.)(Nixon’s Jewish problem. Again.).
7/- Hours after the secretary of State advised his boss to resign for the good of the country, the president ordered him to “cut off all military deliveries to Israel until it agreed to a comprehensive peace.” Nixon added that he “regretted not having done so earlier,” and said his successor, Gerald Ford, would “thank” him.
Kissinger suspected Nixon’s command was “retaliation for our conversation of a few hours ago–on the president’s assumption that my faith made me unusually sensitive to pressures on Israel.” Like others in Nixon’s entourage, Kissinger felt he knew when the president gave an order merely to vent and when he really meant it. The documents to cut off Israeli aid were prepared, but never signed. (A Question Of Anti-Semitism).
8/- In the final stage of Vietnam War, SVN President Thieu gave a resignation speech that revealed detailed reduction of military aids to South Vietnam to almost nothing carried out by Kissinger. Thieu said that Kissinger acted like a fishmonger in a fishmarket (“hàng tôm, hàng cá”).