EVEN IN a city inured to rocking disclosures, the weekend leak of Secretary of Defense Caspar Weinberger's letter to President Reagan was an eye-opener. Here was a powerful Cabinet officer being seen to warn the president against coming to terms with Mikhail Gorbachev on several of the most critical issues ostensibly being considered for negotiation at Geneva. Here was a president five years in office, barely hours away from his first meeting with a Soviet leader, appearing to have failed so far to resolve fundamental issues bearing on summit tactics and grand strategy.

On both these scores -- reluctance in the Pentagon, confusion in the White House -- the letter was an embarrassment to the president and a coup for the Soviets. If Mr. Reagan takes the advice in the letter, he risks being portrayed as a captive of the "military-industrial complex" -- the phrase of President Eisenhower that Mr. Gorbachev has recently put to his own propaganda uses. If the president ignores the advice, it will look as though he has repudiated a faithful aide and an important slice of his constituency. Certainly the leak illuminated public understanding. It also steepened the president's summit path.

The Weinberger letter focused first on suspected Soviet violations of SALT II. In the Pentagon, the letter implicitly granted, there are two views on what military response may be required. The Joint Chiefs feel that programs already requested are enough; the command is on record as arguing that the Soviet Union is considerably better placed than the United States to break through SALT ceilings if the present mutual policy of not undercutting the treaty is set aside. By contrast, the Weinberger view is that Soviet violations require new American strategic programs. To make them legal, he urged the president not to agree to continue observing SALT II.

The secretary also urged Mr. Reagan to rebuff summit pressures to hold to the "narrow and, I believe, wrong" interpretation of the Anti-Ballistic Missile Treaty that the administration recently accepted. To confirm that research-only interpretation in an agreement with Moscow, Mr. Weinberger said, would endanger the president's Strategic Defense Initiative.

In short, Mr. Weinberger would move away from the key existing Soviet-American agreements limiting both offensive and defensive strategic arms. He would end any effort to treat arms control as well as arms building as vital to American security.

Conceivably the summit will finish off arms control, thanks to Soviet intransigence. But the summit should not come to that thanks to American intransigence. If Mr. Reagan's pledge of going to Geneva to make a "fresh start" means anything at all, he will keep his mind open and not allow this latest leak to close it.