At the AIPAC conference in Washington on Monday, Hillary Clinton said of US-Israeli ties: “Our countries and peoples are bound together by our shared values of freedom, equality, democracy, the right to live free from fear, and our common aspirations for a future of peace, security, and prosperity.” But the most important tie is the one Clinton left off the list: weaponry. Weaponry paid for by US taxpayers and sold by the US defense industry, through a military aid program that ensures a steady flow of lucrative contracts to US manufacturers and guarantees that for decades to come the Israeli war machine will remain “made in the USA.”

The diplomatic crisis between the U.S. and Israel has sent a tremor through their alliance, but one key part of the bond seems virtually untouchable: the roughly $3 billion a year in U.S. military aid.

Israel’s harsher critics often call for aid cuts to twist Israel’s arm. Yet amid the uproar of recent days over plans to build 1,600 new homes for a Jewish neighborhood in a disputed part of Jerusalem, there has been no serious talk of using aid as a club.

One reason may be the potential backlash from Israel’s supporters in the U.S. Another is that the overwhelming part of the money cycles back into the American economy.

Israel is the biggest recipient of American aid after Afghanistan. But unlike most other countries, Israel’s aid is earmarked entirely for military spending. Under an agreement between the two allies, at least three-quarters of the aid must be spent with U.S. companies.

This means that the “close, unshakable bond,” as Secretary of State Hillary Rodham Clinton described it, is also a mutually beneficial one: Israel gets the latest American military technology, and American weapons makers – Lockheed Martin, Raytheon, Boeing and others – get a steady stream of income.

The U.S. stepped up funding to Israel after the Arab-Israeli wars of 1967 and 1973, at a time when the Soviet Union was arming the Arabs. Following the 1979 Israeli-Egyptian peace treaty, Washington guaranteed Israel would continue receiving annual military and civilian aid in a 3:2 ratio with aid given to Egypt. Since then, Israel’s share has ranged between $2.1 billion and $3.7 billion a year.

Over the last decade, as Israel’s economy has grown, the U.S. has converted the whole package to military funding, under an agreement to have it at $3.15 billion a year by fiscal 2013 and keep it at that level until 2018.