A high Saudi Arabian defense official has arrived in Geneva, ready to start negotiations with the French government for the Mirage Fl fighter-bomber - showing why President Carter is risking so much to push his sale of 60 F15's to the oil-rich monarchy.

If the sale is vetoed by a vote of both Senate and House, Saudi Arabia will begin to unwind an intricate network of economic and military arrangements with the United States. Switching from Washington to Paris for modern fighter aircraft would be only the start.

What makes that so ironic is that if Israel does succeed in imposing the congressional veto, the ensured supply of the Mirage F1 gives the Saudis at least an equivalent number of modern aircraft. Defense Department experts here are warning that the new Saudi-French connection could erode the U.S. role as long-time military supplier to the Saudi monarchy.

Although putting French instead of American aircraft in Saudi hands would seem to undermine Israeli security, the Israeli government takes a different view. To break up the U.S. - Saudi alliance is worth the risk, in Israel's opinion.

A confidential Pentagon memorandum, written for the State Department, says the Saudis could immediately start buying the Mirage, a highly sophisticated aircraft only marginally less dangerous to Israel than the F15. Yet Israel publicly bases opposition to the F15 sale on the menace to its security.

If the Saudis are forced to turn to Paris, Israel would lose the benefit of U.S. controls over Saudi use of American weapons. There is no reason for France to impose restrictions on the Saudis such as the U.S. ban on transfering weapons to a third country.

Worse yet for Israel's security requirements, there would be no limit on the quantity of Mirages the Saudis might buy. Some highly informed U.S. officials believe the French want to set up a French-Saudi consortium to build more advanced fighters - the Saudis providing the money, the French the technology.

Israel is flooding Congress with security arguments against the F15 sale. Foreign Minister Moshe Dayan last week used his own great prestige here in private sessions with senators, hammering home Israel's vulnerability to the F15. Given the immediate availability of the French Mirage, administration officials suspect that Israel's real purpose in trying to block the sale is not its security but its desire to split the intimate U.S.-Saudi connection.

Defeat of the F15 deal would certainly erode that connection. In addition to advanced aircraft, driving the Saudis into French arms would jeopardize up to $15 billion worth of other military supplies either in the U.S. pipeline or under U.S.-Saudi negotiations. Government officials say that many of those arms can be obtained from France and other European countries; they undoubtedly would be if Congress vetoed the F15 sale.

That would be welcome news for the Israelis. Beleaguered in the Arab world, Israel has long feared the deepening economic and military ties between the United States and Saudi Arabia. A recent unleashing of the pro-Israel lobby to damage that relationship came in the battle to end the anti-Israel Arab boycott though, in fact, the new anti-boycott law was moderate and acceptable to the Saudis.

The battle has now shifted to the F15s, with Jimmy Carter himself on the ramparts to prevent Israel from disrupting the Saudi connection. The intensity of the lobbying campaign - probably the most intense of any Isreaeli effort - has surprised and angered top Carter aides.

For example, the April 19 issue of Near East Report, tone setter for the pro-Isreael lobby, said that "in terms of political recompense for security and assistance provided [to Saudi Arabia], the United States has hardly received fair value." In short, despite Saudi Arabia's heavy financial support for Egypt and other moderate Arab states and its restraint on oil prices, the United States owes nothing to the Saudis.

Carter disagrees. His financial advisers have warned him that erosion of the Saudi connection might well result in shifting some of the multibillion of dollars invested in U.S. government securities and U.S. banks elsewhere and reduce the $5 billion in non-military U.S. purchase programed for 1978.

The president see that unfolding picture clearly. That explains his singular political risk of a head-on confrontation with the organized American Jewish community in a battle that dwarfs the Panama Canal struggle in real importace.