Everyone knows the United States is Israel's best friend. The US gives Israel billions of dollars in aid annually, consistently blocks UN Security Council resolutions condemning Israel, and backs its military offensives publicly. But why? What's the thinking behind America going above-and-beyond for Israel?

The short version: it's complicated. The long version is that It's a tight interplay of America's long-running Middle East strategy, US public opinion/electoral politics, and a pro-Israel lobbying campaign that is effective, but maybe not as effective as you've heard. Here's a guide to the different factors shaping America's Israel policy — and how they relate to each other.

Since the Cold War, Israel has been the linchpin of American Middle East strategy

The US wasn't always so close with Israel. For instance, when Israel (along with France and Britain) invaded Egypt in 1956, the United States sided against Israel, pushing the invaders to leave. And the US for years opposed, and worked actively against, Israel's clandestine nuclear program. "Stated commitments to [Israel from American policymakers] cannot erase a legacy of US policies that often represented more of a threat than a support to Israeli security," Michael Barnett, George Washington University political scientist, writes.

Even when the US did come to support Israel, it was more about cold strategic calculation than the domestic political support you see today. The US-Israel relationship grew "by leaps and bounds" after 1967, according to Barnett, owing largely to "a changing US containment and strategic posture." American presidents and strategists came to see Israel as a useful tool for containing Soviet influence in the Middle East, which was significant among Arab states, and used diplomatic and military support to weave Israel firmly into the anti-Soviet bloc.

This strategic justification came down with the Berlin Wall. Yet the US aid to Israel kept flowing after the Cold War, as did diplomatic support. What kept it going?

the US approach to the Middle East didn't change that much after the Cold War

For one thing, the US approach to the Middle East didn't change that much after the Cold War. The US became increasingly involved in managing disputes and problems inside the Middle East during the Cold War, and it maintained that role as the world's sole super-power in the 90s. Stability in the Middle East continued to be a major American interest, for a number of reasons that included the global oil market, and the US took on the role as guarantor of regional stability.

That meant the US saw it as strategically worthwhile to support states like Egypt, Saudi Arabia, and Israel, which saw themselves as benefitting from an essentially conservative US approach to Middle Eastern regional politics. Unlike, say, Iran, Syria, and Saddam's Iraq, these countries were basically OK with the status quo in the Middle East. The US also supported the status quo, so it supported them accordingly.

This view of Israel as a "force for stability" helps maintain US support, according to Brent Sasley, a political scientist at the University of Texas, "in the sense that Israel can stabilize what's going on in the Middle East. If there's fear of Jordan being undermined by an internal or external enemy, the United States sometimes turns to Israel to pose a threat to that threat."

America's self-appointed role as manager of the Middle East also landed it the job of Israeli-Palestinian peace broker.

"The parties need a third party," Hussein Ibish, a Senior Fellow at the American Task Force on Palestine, says. "I think there is no other candidate than the United States. There's no other party that's capable, and no other party that's interested."

American policymakers have seen US support for Israel as a way of showing Israel that the US is still taking its interests into account during negotiations, and thus convincing Israel that they can safely engage in peace talks. It's meant to draw the Israelis to the negotiating table, and keep them there.

Together, these strategic factors explain why America's approach to Israel has been broadly consistent for at least the past three administrations. Despite the vast disagreements between the George W. Bush administration versus the Clinton and Obama administrations on foreign policy, they've both supported military and political aid to Israel. And they've both crossed Israel when it wasn't in the US' strategic interests: Bush refused to support an Israeli strike on Iran, and Obama repeatedly clashed with Israeli leaders on West Bank settlements.

All of this isn't to say that American presidents and foreign policy principals are necessarily right to believe these things. It's within the realm of possibility, as some argue, that US support for Israel undermines regional stability and compromises America's status as neutral broker during peace negotiations. The point here isn't to endorse the official US view, but describe the line of thinking that's been so influential in driving the American foreign policy establishment's approach to Israel.

Supporting Israel is good politics in the US

US support for Israel isn't just about strategic calculation and foreign policy interests, or at least not anymore. For a long time, at the very least since the 1980s, it's also been about domestic politics and the way American politicians read American voters.

Congressional votes on issues relating to Israel are famously lopsided. The Senate resolution supporting Israel's recent offensives in Gaza passed unanimously, as many "pro-Israel" bills and resolutions do.

The simplest explanation for these lopsided votes is that supporting Israel is really, really popular among voters. "The single factor most driving the U.S.-Israel relationship appears to be the broad and deep support for Israel among the American public," Israel Institute program director Michael Koplow writes. "The average gap between those holding favorable and unfavorable views of Israel over [the past four administrations] is 31 points."

Indeed, Gallup data since 1988 consistently shows a much higher percentage of Americans sympathizing with Israelis than with Palestinians in the conflict:

So it makes sense that Congresspeople would take pretty hard-core pro-Israel stances: it's reasonably popular.

But why is Israel so popular among Americans in the first place? One big reason is a perceived sense of "shared values." According to Barnett, the American moral image of Israel — "the only democracy in the Middle East," for example — is the "foundation of US-Israeli relations." Of course, as Barnett hastens to add, this leaves Israel vulnerable if Americans comes to believe that Israel has strayed from those shared values (more on that in the last section).

Religious groups are two other critically important factors. American Jews and evangelical Christians are two of the most politically engaged groups in the United States. They're major constituencies, respectively, in the Democratic and Republican parties. And both are overwhelmingly pro-Israel.

There are nuances here: evangelical support for Israel tends to be more uncritical than Jewish support. For instance, a majority of reform and secular Jews — 65 percent of the American Jewish population — disapprove of Israel's expansion of West Bank settlements. And Jews under the age of 35 are the least likely to identify as Zionist (though a majority still do). On the other hand, the older and more conservative Jews who aren't entirely representative of the more liberal body of Jewish-American public opinion toward Israel, have a lot of clout with national politicians. They express strong desire to vote based on the Israel issue and are clustered in Florida and Pennsylvania, large swing states in presidential elections.

All that said, Pew data shows overall consistency in American Jewish views on the US-Israel relationship. 54 percent of American Jews think the US supports Israel the right amount — and 31 percent say it doesn't go far enough. By contrast, 31 percent of white evangelicals think the US has reached the right level of support, while 46 percent want the US to support Israel more.

Add evangelicals, Jews, and broad public support together, and you get consistent, bipartisan support for Israel.

There's also a huge pro-Israel lobby — but how effective are they really?

No account of US-Israel relations can ignore the American Israel Public Affairs Committee — AIPAC for short. AIPAC is America's largest pro-Israel lobby. Surveys of Capitol Hill insiders conducted by Fortune (1997) and National Journal (2005) ranked it the second-most powerful lobbying shop in Washington, after (respectively) the AARP and National Federation of Independent Business. Neither survey is particularly statistically rigorous, so don't take the specific rankings too seriously. And AIPAC loses on plenty of issues. However, the surveys do suggest that AIPAC is perceived as hugely powerful within Washington.

Saying that AIPAC pushes US foreign policy in a more pro-Israel direction isn't controversial. The big, and extremely contentious, question is just how much AIPAC actually matters. Is the group actually steering US politics and foreign policy in a direction it wouldn't go on its own?

AIPAC is an extremely influential lobbying group, but its power is linked to the other sources of US support for Israel

The major flashpoint here is John Mearsheimer and Stephen Walt's The Israel Lobby and American Foreign Policy, which began as an 2006 essay and evolved into a book. The two eminent international relations scholars argued that there's no way to explain the US-Israel relationship, from an IR perspective, other than as AIPAC and its allies pushing the US to act counter to its own interests. They reject that either strategy or shared values fully explain the US support for Israel, so lobbying must. "The unmatched power of the Israel Lobby," Walt and Mearsheimer write, is "the" explanation for America's continued strong support for Israel.

This argument is hugely controversial, including among international relations theorists. Some argued that The Israel Lobby creepily invoked classic anti-Semitic tropes of Jews secretly controlling the government. Others dismissed it as, in one particularly memorable phrase, "piss-poor, monocausal social science."

One of the main criticisms of Walt and Mearsheimer's thesis is that they don't present very much direct evidence that AIPAC lobbying influenced specific votes. Another criticism is that Walt and Mearsheimer premise their thesis on the argument that Israel is neither strategically nor morally worthy of American support, and so policymakers must be supporting Israel because they've been coerced into it by AIPAC, whereas a number of policymakers will tell you they earnestly believe the alliance is worthwhile absent lobbying. Critics also argue that the definition of "Israel Lobby" beyond AIPAC used in the book is so large as to encompass basically the entire American foreign policy establishment.

Whatever you think of this debate, it can be easy to get lost in a binary between "the Israel lobby is all that matters" and "the Israel lobby is irrelevant." What's clearly true is that AIPAC is highly influential, but also that its power is linked to the other sources of US support for Israel; it does well on whipping up support for bills that are already in line with public opinion.

AIPAC doesn't always win. For instance, it lost a major fight in Congress when it pushed for more sanctions on Iran in February 2014; the sanctions were likely designed to kill the ongoing US-Iran nuclear negotiations. AIPAC's influence is a product of financial resources and power, sure, but also of choosing to push for policies that have public support and are consonant with American grand strategy in the Middle East.

Could US support for Israel change?

It's hard to know where one driver of America's Israel policy ends and another begins. For instance: early in his administration, President Obama pushed Israeli Prime Minister Benjamin Netanyahu to halt settlement growth in the West Bank; Netanyahu resisted this in part by rallying his allies in Congress. Netanyahu's allies in both parties, who are always eager to appear pro-Israel, pressured Obama to drop his anti-settlements push, which he did.

The question here is whether, in this case and others, US foreign policy interests or US domestic politics was ultimately more consequential to driving the US-Israel relationship. For example, would Obama have pushed harder against settlements had Netanyahu not been able to call up so many allies in Congress? Were those members of Congress primarily driven by pure domestic politics, which do favor pro-Israel policies, by an earnest concern that Obama's approach was bad for Israelis, or by a belief that Obama was hurting US foreign policy interests?

In thinking about the future of US-Israel relations, it's much more helpful to examine what might cause these broad-bush factors to change. In simpler terms: is there a scenario under which the US and Israel drift apart?

"US-Israeli relations are dependent upon Israel's having a particular identity"

Barnett, the George Washington University scholar, sees Israel's continued occupation of the West Bank as the greatest threat to the relationship. He notes that, in the early '90s, Congress made a $10 billion loan guarantee conditional on the fact that Israel didn't use any of the money for West Bank settlements. Israel's prime minister, Yitzhak Shamir, tried to fight it, but the Bush administration stood firm. Shamir lost, both in Congress and with the executive, because the Israeli position wasn't consistent with the US vision of a Western, democratic Israel.

"US-Israeli relations," Barnett writes, "are dependent upon Israel's having a particular identity." That may even be true among American Jews, as journalist Peter Beinart argued in an essay almost as controversial as Walt and Mearsheimer's. Beinart argues that Israel's ongoing occupation of the West Bank is already alienating younger and more secular Jews, and that AIPAC and other mainstream Jewish organizations risk losing their broad base of support unless they become more willing to criticize Israel on these points.

Barnett's conclusion only follows if you think "shared values" are the linchpin of US-Israel relations. Maybe the US would still think it's strategically useful to support Israel. Maybe Israel remains popular among certain Christians and the broader public regardless of its Palestinian policy. Maybe AIPAC remains strong enough to keep Congress in line. Maybe Israel comes to an agreement with the Palestinians and Barnett's point becomes moot.

For now, though, there's little evidence that American support for Israel is fundamentally breaking down — whether you think that's a good or bad thing.