Why US foreign policy is pro-Israel? What benefit does the US get out of this alliance?
Israel is the closest ally of the USA in Arab world. USA has always showed its unconditional support for Israel on many occasions.American Senator Jesse Helms used to call Israel, "America's aircraft carrier in the Middle East." We have to understand the reason of this much support to Israel by the USA, but before that first we must dispel two misconceptions, or even myths: A) The U.S. supports Israel because of the Holocaust. Countries, the U.S included, do not make foreign policy choices based on sympathy, surely not sympathy for things which occurred over half a century ago. Governments act according to what they perceive to be their best interest (the short list of cases in which the U.S. orchestrated a "Humanitarian intervention" vs. the long list of cases where such a thing was called for and never happened is a good demonstration of this rule).
B) The power of the Jews in the U.S: The Jew's political power in the U.S., though certainly present, is highly overestimated. True, over 6 million Jews live in the U.S., however, one must remember that one is considered a Jew based on their ethnicity, not their religion or sympathy to the Jewish people. Studies show that over half of the Jews in the U.S. are wholly unaffiliated, meaning that their Jewishness is of no significance to them. There are many wealthy Jews in the U.S., who all engage in philanthropy, but many of them don't give a dollar to Jewish causes. Of those Jews who do affiliate with Judaism, not all support Israel, and some are outright against Israel. Many Jews in the U.S are fully against using political influence in order to promote Israeli causes. They see this as insincere and undemocratic. Of the remaining, there are many who are very active politically who try to influence the U.S's position towards Israel. They employ a very strong lobby known as AIPAC (American Israeli Public Affairs Committee). These may have an influence on the particulars of the U.S policies towards Israel, but by no means do they have the power to determine the fact of whether or not the U.S maintains its allegiance to Israel, and could not hold up if it were no longer in the U.S's best interest to do so.
Now that we're done with what is myth, let's turn to some history: In the wake of WWII, two very large and very strong countries were on the verge of becoming super powers - the U.S and the U.S.S.R (soviet Russia). They both realize that the way to consolidate and increase their power is to gain as much possible control over international relations by means of collecting as many ally and proxies as possible. At the same time, two former super powers which had exercised influence over most of the world were disintegrating - France and the UK. They were being kicked out of their provinces, and the U.S. and Russia wished to enlist as many of the new independent countries rising in the aftermath to their side.
The Middle East was split between France and the UK. However, Russia moved in quickly and efficiently into the Middle East after they left, and bought the alliance of virtually all the Arab countries. This is evident from the fact that in 1948, when all these Arab countries attacked Israel in what became its independence war, they were all bearing soviet arms (Israel was using European arms, which they procured mainly from the Czech Republic). America had no proxy through which to exert influence in the Middle East. When the U.N voted on the partition plan (which recommended giving half of Palestine to the Jewish people and half to the Arabs) both the U.S and Russia voted for it. This was unprecedented and to the best of my knowledge has never happened since. This can only be explained by the fact that both were debating endorsing the Jewish state.
However both Russia and the U.S were suspicious of Israel, and were not inclined to endorse them at first. The Russians didn't want to aggravate their Arab allies. The U.S on the other hand suspected Israel of having leanings towards Russia. This suspicion was not unfounded. Most of Israel's early leaders came from Russia and Eastern Europe and had ties there, and many of them wished to court Russia's allegiance over the U.S. Israel's prime minister, David Ben Gurion insisted however that Israel try to create ties with the U.S. (they say that this is mainly due to the U.S's democratic nature). The tipping point occurred when Israel managed to obtain a recording of a secret speech given by the Russian leader Khrushchev in 1956 and gave it to the U.S as an act of good faith. Only in the 1960's was the first real aid received from the USA after the six day war of 1967. In 1972, following Egypt's second defeat by Israel, the U.S managed to broker a deal for peace between the two by means of promising aid to Egypt, hence efficiently buying Egypt away from the soviets. The only thing sustaining the peace between Israel and Egypt is the formers dependence of U.S aid, despite enormous animosity in Egypt towards Israel. In many ways, Israel was the gateway for the U.S into the Middle East.
The US is picky about the countries it supports, and it prefers supporting countries which have proven their strength (especially military, but also economic etc.) even without strong American support Israel did this throughout history; especially in 1967. The US has been burnt more than ones by supporting forces that can’t sustain themselves without American support (or even with it), e.g. anti-Communist forces in Cuba, South Vietnam, the “democratic” government in Iraq, etc.
Many believe that only Israel benefits from the strong business relations between the two countries, but this could not be further from the truth. Israeli businesses have a very strong presence in the US, which provides Americans with jobs. It was just recently published that Israeli companies contributed $9 billion/year to the economy of Massachusetts alone. Moreover, the military ties between the two countries keep many peripheral manufacturers in the US afloat. Most countries subsidize their peripheral areas, and one way the US does this is by sending money to Israel that’s then spent in such places in the US.
Israeli society is cohesive, and it is one of the very few countries which have never been non-democratic. Israel has very strong national institutions, and is very unlikely to fall into chaos. This is also true for other strong US allies like UK, Canada and Australia, but less true for most countries that like the US from the poll linked above. Israel is certainly the most stable country in the region by far, including other regional allies like Jordan and Turkey.
Israelis speak Hebrew or Arabic as a native language, English is widespread and an estimated 85% of Israelis speak English on an acceptable level (much more than in Italy or South Korea, for example). This means that Israeli culture is heavily influenced by English-speaking culture, especially American. On the flip side, Jewish culture is very strong in the US. Moreover, Israeli culture is in many ways Western.
In sum, the U.S supports Israel because it is in their best interest, or in the least perceived by them to be in their best interest. The US–Israel relationship is strong and possibly unique. As Michael Oren said, to the US, Israel is the ultimate ally.