President Carter will run into some harsh military realities if forced any time soon to fulfill last night's pledge to protect "the Persian Gulf region" by "any means necessary."

He has ordered the Pentagon to organize a 110,000-man Rapid Deployment Force (RDF) designed expressly to fight in such distant places as the Persian Gulf.

Also, his new defense budget calls for developing a fleet of CX cargo planes to deliver the heavy tanks and other armor such a force would need against Soviet weaponry.

But both the RDF and the CX are still on paper, with 1983 the earliest that the spearhead of the special, long-distance reaction force would be ready for battle.

In the meantime, the president could send units already on hand to the Persian Gulf if U.S. interests are threatened. However, this would be difficult.

Getting there would be one problem. Finding a launching pad for U.S. forces would be another. And sustaining any kind of major war there would be a third.

The only planes large enough to carry the Army's big tanks to the Persian Gulf region are the Air Force's fleet of 77 C5As. But their wings are weak, limiting how intensively they could be used.

Troops could be flown to the Persian Gulf in everything from C141 military transports to passenger airliners, assuming refueling bases would be available along the way.

Assault ships carrying Marines and cargo vessels loaded with war supplies also could steam to the Persian Gulf as soon as Carter passes the word, but the 12,000-mile journey from the United States would take weeks.

If the situation did allow the weeks needed to get the groops and equipment to the Persian Gulf, there are no big American bases to serve as staging areas for those forces. The United States used to have big bases in such countries as Libya and Saudi Arabia, and, until the fall of the shah, it assumed it could use air and naval facilities in Iran.

Gen. Lew Allen, Air Force chief of staff, said recently that fighter-bombers need to be within 300 miles of the target area to make full use of their power. The United States has no base close to the Persian Gulf, while the Soviets are near at hand.

Aircraft carriers can steam close to the gulf, or even into it, to launch their fighters-bombers. But defending the Persian Gulf region, as Carter promised to do last night, means being prepared to put troops on the ground at any crucial moment.

And if, despite a lack of staging areas, the United States did manage to land troops on the ground and they suffered heavy losses, today's Army is short 100,000 reservists needed as replacements.

Carter's decision to seek registration of draft-age youths came after the Joint Chiefs of Staff said this step is vital to gearing up for war.

Those are some of the realities that confront the president and underlie the oft-stated complaint of military leaders that they are short of "lift" -- the ships and planes needed to deliver troops and weapons from the United States to a battle zone.

Carter's new defense budget testifies that these warnings have been heard. It calls for a start on a $6 billion program to build a fleet of cargo planes to replace the C5A; $3 billion for ships full of military gear that would be anchored near trouble spots, and billions more for assault ships for the Marines and other weaponry for long-distance fighting.

Both civilian and military leaders at the Petagon conceded that the planes and ships for "lift" lost out in past budget battles to the glamor weapons such as bombers and aircraft carriers. They also agree that the American military presence must be strengthened in "the Persian Gulf region," which presumably extends from East Africa to Iran to Pakistan.

So Congress, as it reviews Carter's new plan for projecting U.S. military power to remote regions, will find its civilian and military witnesses marching in lock-step on the hardware issues.

Similarly, the Joint Chiefs agree with their civilan superiors that the U.S. military presence must be kept in low profile in Third World countries. Nobody is talking about building another Cam Ranh Bay, the huge port the United States built in Vietnam to support military operations there.

"Third World countries wouldn't let us do that even if we wanted to," said one general. "Access is the key. If we're asked to come in, we want our planes landing on bases flying the host nation's flag, not ours."

State Department and Pentagon officials took that message to Kenya, Somalia, Saudi Arabia and Oman recently. Leaders of those countries consider it politically dangerous to appear overly friendly to the United States.

Egypt and Israel are not so inhibited. The Pentagon executive said Egyptian President Anwar Sadat is proving to be the key in implementing the Carter blueprint for increasing U.S. military power in East Africa and the Persian Gulf. One Pentagon hope, perhaps overly ambitious, is that Sadat will enlarge the Suez Canal to the extent that the largest ships of the U.S. fleet will be able to sail through it, thereby connectig task forces in the Mediterranean Sea and the Indian Ocean.

However, the realities of military confrontation and possible conflict with the Soviet Union already are generating differences of opinion as administration officials try to implement Carter's master plan.

Navy leaders, for example, argue that the United States cannot assume that the fleet of prepositioned cargo ships stationed off world hot spots to resupply the Rapid Deployment Force would not be attacked.

Even a second-rate navy, never mind the Soviet one, could sink the unarmed floating arsenals the president wants to build, according to some military leaders. So, goes the argument, the Navy will have to protect them.

But the Navy is so short of warships that it had to take one of two aircraft carriers out of the Mediterranean to cover the Indian Ocean. The additional job of protecting the prepositioned cargo ships would make a bad situation worse, in the view of Navy leaders.

The same situation of robbing Peter to pay Paul confronts the Marine Corps as it examines the plan to station a force of amphibious assault ships in the Indian Ocean to replace combatants now plying those waters.

The Indian Ocean mission will mean reducing the forces of assault ships in the Pacific and the Atlantic, perhaps heightening the fears of the Japanese and others that the United States is turning its back on the Pacific to concentrate on the Persian Gulf.

Another controversy is building over the question of stationing Marines on the assault ships in the Indian Ocean. Civilian leaders see little sence in asking Marines to sit in the 100-degree weather for weeks on end. The more sensible course, in their view, would be to assign a skeleton crew to run the assault ships and then fly the Marines for them to the island of Diego Garcia only in periods of tension.

The contrary military view is that the Marines should be aboard "so we could punch our way ashore," in the words of one defense official. Anything less, goes this argument, would not look like a credible threat.

No matter which side prevails, the island of Diego Garcia in the Indian Ocean is in for a major expansion so it can handle the biggest planes and aircraft carriers, billet troops and store tons of gear. One Pentagon executive said the price tag could end up being five times the $170 million already earmarked for the island's improvement over the next five years.

What all this adds up to is that Carter has charted a new course for the American military. But the objective -- if Congress goes along with policing distant countries, as the president has recommended -- will take years to reach, given the realities of organizing forces and building new planes, ships and other weaponry.