The United States backs Israel only tentatively. Jewish representatives, not US officials arranged the votes of negligible countries for the UN resolution to establish Israel. In 1947, America did not know or care much about the Middle East. The oil flow was smooth, and no one imagined that someday the US would have to placate Muslims to get oil. The US establishment, culturally and financially close to the Jews, acquiesced in establishing Israel but offered no practical help.
After the Arabs promised to drown the Jews in the sea if Israel were established, America pressured the Jews not to declare their state. The United States fought in many post-WWII wars and supported one side or the other in scores of other conflicts. Israel got no help from America in the 1947-48 war.
The Israeli War of Independence was protracted and bloody. America had plenty of opportunities to intervene. It did not. A little later in 1956, America applied diplomatic pressure and threatened military action to stop an Israeli-Franco-British strike against Egypt. Both Israel and Egypt were economically irrelevant to America, but it helped Egypt, not Israel. America has not hesitated to support colonialist powers against their vassals: France received US aid to fight in Vietnam. Egypt and Vietnam had similar communist regimes, but America supported the Egyptians. Its attitude to Egypt stems from a built-in dislike of Jews. The US establishment does not hate Jews enough to exterminate them but is sufficiently contemptuous of Jews to neglect helping them even in grave danger, whether by bombing Nazi death camps or supporting the newborn Jewish state against six regular Muslim armies.
Post WWII shock cannot explain America’s failure to help Israel. The US jumped into a conflict with the USSR in 1948 over the West Berlin blockade. America risked escalation with an ally over a minor problem with a former enemy. America stood behind the Germans but not the Jews.
The US attitude to Israel changed concurrently with its attitude to China and for the same reason: to counter the Soviets. Dissatisfied with insufficiently orthodox Israeli socialism, her affinity with France, and driven by Russian anti-semitism, the USSR abandoned Israel and stood by the Muslims. The US responded by supporting Israel. The American rapprochement with Israel was a one-man—Kissinger—show, not a culturally or politically predetermined development. In fact, many high-ranking American military opposed cooperation with Israel.
American assistance to Israel has always been meager. It was altogether negligible before 1971, increased in the closing days of the 1973 war, and peaked shortly after the Camp David capitulation. Inflation and the rising cost of the American weapons Israel bought made the assistance insignificant. The aid was important only to big Israeli brass who boosted their egos with expensive, unnecessary military toys from the United States. The changes in aid level show that the US was most interested in Israel at the peak of her strength after the 1967 war, less interested during the cold confrontations with the Soviets in the mid-70s, and uninterested when Israel showed her weakness by giving the Sinai to Egypt.
America, as usual, veiled its realpolitik support of Israel in moral terms, and the moralizing stuck. The aid, eroded by inflation, went on long after its rational basis evaporated.