A Persian Gulf war would provide the most challenging test yet of the all-volunteer, $3 trillion U.S. armed forces enhanced by a massive infusion of Reagan administration dollars and molded by the mistakes of its last war in Vietnam.

After five months of frenzied preparation and organization in the Saudi desert and surrounding seas, the U.S. military has massed 415,000 troops in the gulf region. As the United Nations' deadline for Iraq's withdrawal from Kuwait expires at midnight tonight, perhaps the most critical issue confronting military commanders is the readiness of their troops and equipment to meet 540,000 Iraqi troops and their complex network of battlefield fortifications.

Although most military commanders are confident that the United States has assembled a fighting force that can defeat the battle-hardened but less sophisticated Iraqi war machine, questions over the degrees of military readiness remain.

Positioned on ships, in planes and across the sands, the U.S. military is prepared to carry out potential presidential orders for air strikes against Iraq, according to Pentagon officials. But it still is scrambling to put in place the ground forces that commanders say they need to fully counter Iraq's potent armor and infantry forces in combat.

Although 245,000 Arab and allied forces have joined the U.S. troops in the region, military officials say if there is war, the primary battle action will involve Americans and Iraqis.Army's Readiness Is Crucial Question

The Air Force's arsenal of bombers and fighters, ranging from the barely tested F-117A "stealth" fighter to the war-hardened B-52 bomber, is largely in place and ready for its planned role as the dominating force in the early days and weeks of any attack against targets in Iraq and occupied Kuwait.

The Navy has rushed three more aircraft carrier battle groups to join the three already patrolling the Red Sea, northern Arabian Sea and Persian Gulf. Two of the additional carrier groups have just arrived in the region and a third is expected to transit the Suez Canal today and arrive in the Red Sea. The additional carriers, which will supplement the military's air power in the region, need several days to orient pilots and crews to the operating environment in the Middle East, officials said.

The most controversial aspect of military readiness revolves around the troops that would bear the brunt of any all-out war with Iraq: the ground troops, tank drivers and field commanders of the Army and Marine Corps.

While military officials say 415,000 U.S. troops were in the region as of yesterday, authorities said only 370,000 of them are positioned in their wartime locations and are fully prepared for possible combat. The others remain at varying stages of preparedness ranging from fresh arrivals to those just moving into the desert to those only now digging their foxholes and tank bunkers in the sands.

The troops that have been maneuvering and exercising in the desert since last autumn now have a substantial advantage over new arrivals, officials said.

Readiness, according to Barry R. Posen, a Massachusetts Institute of Technology military analyst, "means different things to different people." He said a critical ingredient is "the transition they have to make . . . getting desert-smart."

Across the border in southern Kuwait and Iraq, according to Gen. H. Norman Schwarzkopf, the senior U.S. commander in the gulf, the greatest strength of the enemy forces is "just their mass" -- now 540,000 troops organized into more than 30 divisions in the theater of operations. But Gen. Colin L. Powell, chairman of the Joint Chiefs of Staff, said Thursday that the Iraqis' fighting prowess is uneven.

Some parts of the Iraqi force, Powell said in an apparent reference to five elite divisions of the Republican Guard, "are very competent, very well led, paid more, get the best equipment." Other units, he said, "are much, much less capable," referring to the first-echelon infantry divisions dug in at the Kuwait-Saudi border. New conscripts sent to the front lines since November, he said, "are even less capable."

Iraqi weapons strength is mainly in armor and artillery. There are about 4,000 Iraqi tanks in the theater of operations, about 500 of them top-of-the-line Soviet T-72s that are considered on a par with the best U.S. tank, the M1-A1 Abrams. Iraq also has deployed about 2,700 armored personnel carriers and infantry fighting vehicles and 3,000 artillery pieces, including 300 long-range South African weapons that fire powerful 155mm shells. While many modern munitions are designed primarily to destroy other weapons, artillery's principal purpose is to inflict casualties, and Iraq used its firepower with devastating effect in its eight-year war with Iran.

Iraq's Scud-B ballistic missiles, the subject of much public speculation, are regarded by U.S. commanders primarily as weapons of terror rather than serious threats to military facilities and troops because they are inaccurate and carry modest payloads of explosives.

The Iraqi navy is negligible and its 500-plane air force is greatly overmatched by American-led air forces, in both number and quality. The best of the Iraqi planes are about 94 French-made F-1 Mirages, some armed with air-to-ground Exocet missiles, according to the International Institute for Strategic Studies. Many U.S. officers and independent analysts expect the Iraqi air force to be incapacitated within one to three days after aerial combat begins. Allies Count on Controlling Skies

Allied forces, in fact, are counting on control of the air to counter Iraqi armored strength. U.S. and allied combat aircraft number 1,300, and commanders intend to use fighters and ground-attack planes to destroy their Iraqi counterparts and Iraqi air defenses so that tank-killing A-10 attack planes can be unleashed against Iraqi armor.

While the Arab and other allied forces assembled in Saudi Arabia represent important political demonstrations of support, U.S. military commanders say these forces will contribute only a small percentage of the military force needed in any attack against Iraq.

The most critical allied military forces, officials say, are the British combat troops now teamed with the U.S. Marines and British fighter and attack jets, as well as the Egyptian armored forces that are arrayed on the western flanks of allied forces near the Kuwaiti border and that would be used to help stop an Iraqi thrust into Saudi Arabia.