Even if President Carter's heralded Rapid Deployment Force for the Persian Gulf region was all ready to go to war, its backers and critics agree that it could not be used to end the Iranian-Iraqi conflict.

Why this is so is a story of military realities that are inhibiting such direct actions as "sending in the Marines," thereby breeding political frustration in the United States.

"This is the 1980s, "said one military leader yesterday in making the point that the Iranian-Iraqi war is the kind of regional conflict that the United States, with or without the Rapid Deployment Force, will have to sit out.

Yet, Pentagon planners predict there will be a series of such conflicts for the foreseeable future, many of them threatening oil needed to fuel the economies of the United States, NATO allies and Japan.

The Rapid Deployment Force, the designation for several existing Army, Navy, Air Force and Marine units assigned to rush to distant trouble spots like the Persian Gulf to influence the situation, gained impetus after the Soviet Union invaded Afghanistan.

Congress has since approved down payments on what will be a multibillion-dollar investment in the RDF, including speedy ships, transport planes and improvement of existing ports and airfields in Oman, Egypt, Somalia, Kenya and the British island of Diego Garcia in the Indian Ocean.

The idea of the bases is to put American military power closer to areas of vital interest, such as the oil-rich Persian Gulf and East Africa. aBut the Iranian-Iraqi war dramatizes why it must be relegated at times to an "invitation only" outfit, even if this means losing oil.

Neither Iran nor Iraq is asking the United States to help. In fact, both countries are blaming Washington for inciting the warfare. So, as Pentagon planners see it, the United States would intervene in this regional conflict only if the Soviet Union did so.

But U.S. military leaders recoil from the idea of taking on the Soviets in their own backyard. The U.S. forces would be at the end of a 10,000-mile supply line, one that could be cut.The RDF, in short, is not on the ground in strength anywhere near the Persian Gulf today -- and will not be for years.

The growing appreciation of this reality helps explain why President Carter and congressional leaders are looking beyond intervening in the Iranian-Iraqi conflict and concentrating on such other options as these:

Organizing a multinational naval force to keep vital waterways open, such as the Strait of Hormuz off Iran.

Getting NATO partners to fill the gaps opened up in Europe as U.S. troops respond to trouble in the Indian Ocean theater.

Increasing the size of the U.S. military.

Putting more military equipment in staging areas near Saudi Arabia, a vulnerable country considered vital to U.S. interests.

At the same time the government is assessing those options to enable it to do more militarily in the Persian Gulf, Congress is expressing some second thoughts about getting too entangled in doubtful countries, with Somalia the immediate case in point.

Long before yesterday's promise by President Carter to explore with other nations ways to keep vital waterways open to oil tankers, a group of House members were pushing for such a step.

Rep. Paul Findley of Illinois, ranking Republican on the House Foreign Affairs Middle East subcommittee, and 29 other representatives have introduced a resolution to organize a Sealane Security System comprised of navies of the United States and friendly nations.

The threat the Iranian-Iraqi war poses to the Strait of Hormuz, Findley said yesterday, together with the inability of the U.S. military to influence events on the ground, dramatizes the need for such a multinational approach to sealanes.

If Japan, which depends on oil from the Persian Gulf, cannot contribute warships to such a force. Findley said, "let them be the shipbuilders for the alliance. They're getting a free ride, and they know it."

Representatives who recently joined Findley in sponsoring the sealane resolution include Chairman Melvin Price (D-Ill.) of the House Armed Service Committee.

The U.S. Navy, while saying it still

Findley and his allies are pressing the Carter administration to insist that other oil-importing nations join the United States in any Persian Gulf military effort before a single soldier from the Rapid Deployment Force is put on the ground.

Although this is an appealing idea, Pentagon planners caution that it may not always be feasible. If a coup threatens to topple the pro-U.S. leadership of Saudi Arabia, one military official said yesterday, there probably would not be enough time to organize any kind of multinational intervention.

Sen. Henry (M. Jackson (D-Wash.), while agreeing that the United States must be prepared to take unilateral action to save the "very vulnerable" Saudi Arabia, complained there is no contingency plan for using the Rapid Deployment Force or any other military outfit there.

"I keep asking what do you do if there's a coup in Saudi Arabia," Jackson said. "What is the plan? I've asked this at the highest levels. There is no plan."

The Pentagon, as part of its fiscal 1982 budget plans stamped secret, has been discussing reimbursing West Germany for the cost of activating reserve units to take the place temporarily of U.S. troops sent on exercises or missions to the Persian Gulf area.

Such a plan, said Sen. John G. Tower (R-Tex.), a member of the Senate Armed Services Committee, "will never fly" in Congress. He said West Germany's economy is in better shape than that of the United States. Also, taking American troops out of NATO, even temporarily, would touch off jitters in the alliance, Tower predicted.

"I don't see why, however, that West Germany can't put more in the GIUK gap," Tower said, referring to the ocean gap between Greenland, Iceland and the United Kingdom, where NATO forces monitor the movements of Soviet submarines.

Rep. Jack Edwards of Alabama, rankin Republican on the House defense appropriations subcommittee, said U.S. forces cannot be stretched to cover the Persian Gulf. He contended only a bigger U.S. military would make the Rapid Deployment Force a credible threat.

Edwards said: "What the Iranian-Iraqi thing says to me is: 'Here's another warning. You better start getting ready.'" The congressman vowed to spend next year's subcommittee budget hearings hammering away at the unmet needs of the Rapid Deployment Force. "We need some hard facts," he said, "like what kind of ships they're going to buy."

Sen. Gary Hart (D-Colo.), a member of the Senate Armed Services Committee, complained in another interview that the Rapid Deployment Force has the same complicated command structures that helped doom last April's effort to rescue the American hostages in Tehran.

Rather than try to combine the Army, Navy, Air Force and Marine Corps in one Rapid Deployment Force, said Hart, "give the role to the Marine Corps. That's always been its traditional role."

Jackson and Sen. Sam Nunn (D-Ga.) stressed that despite the flaws in the Rapid Deployment Force, it is a step in the right direction.

The Pentagon planners have to put more emphasis on protecting Saudi Arabia by pre-positioning military equipment as near its borders as possible, with the base rights in Oman and Ras Banas, Egypt, a good start, Jackson said.

Nunn, declaring that "the last thing we want to do" is send the Rapid Deployment Force into the current regional conflict between Iran and Iraq, said the United States must continue working on ways to give policymakers more military options in the Persian Gulf region.

Chairman Stephen J. Solarz (D-N.Y.) of the House Foreign Affairs Africa subcommittee said the Iranian-Iraq war "underscores that regional rivalries and ethnic hostilities" are likely to drag the United States into conflicts it cannot resolve. This is giving Congress second thoughts about approving administration plans to establish a military presence in Somalia, Solarz said.