The catastrophic terrorist bombing of the U.S. Marine compound in Beirut yesterday produced a gruesome new crisis for U.S. foreign policy at a time when other aspects of the Lebanese problem and an unusually long list of dangers elsewhere in the world have stretched thin America's diplomatic and military resources for dealing with them.

The blast also caught the Reagan administration at an awkward moment, with key policy-makers changing jobs at almost every level concerned with the latest crisis. Included are a new White House national security affairs adviser, a yet-to-be-named new special Mideast negotiator, a new U.S. ambassador to Lebanon and a new assistant secretary of state for Near Eastern affairs.

On Friday, Oct. 14, and again last Tuesday, President Reagan and the top echelon of his foreign-policy team met in the situation room of the White House as the National Security Planning Group to survey the course ahead in Lebanon and the Mideast.

A tangible result of the discussion, according to administration sources, was a presidential decision last Tuesday that the Marines' mission and personnel strength in Beirut should remain unchanged, despite persistent Pentagon reluctance to leave the troops in their exposed positions.

The policy-makers' concern had been heightened by a new threat that developed shortly before the Oct. 14 meeting: for the first time, the Marines in Beirut were conscious and definite targets of attack, in this case by sniper fire.

The sniper threat, which took the lives of two Marines a week ago, was different from earlier shelling in which U.S. troops were caught in firing that might not have been aimed specifically at them.

Although as late as Friday the State Department refused to acknowledge publicly that targeting of the Marines created a new, more hazardous situation, officials said yesterday that U.S. diplomats were active in recent days in seeking to investigate and diminish the threat attributed to radical Shiite Moslem groups.

Despite the ominous development on the ground, there was optimism toward the end of last week that the long-delayed political process of reconciliation among contending groups in Lebanon will begin in Geneva within a few days.

The administration feels strongly, officials said yesterday, that this should proceed and that there is no reason to expect that terrorist killings of Marine and Navy personnel and of French forces will interfere.

For the U.S. government and people, the death of so many servicemen in one day is a catastrophe but, in the killing ground of modern Lebanon, the death toll is not startling.

About 50,000 Lebanese and Palestinians, mostly civilians, were killed in the seven years of civil war following 1975, according to rough estimates. Another 17,000 were reported killed last year following the Israeli invasion of Lebanon, and the casualties continued this year in new battles among Lebanese groups.

The basic results of the still-incomplete White House policy review, as far as Lebanon is concerned, were to be announced in a speech by Deputy Secretary of State Kenneth W. Dam to the American Jewish Committee Thursday night in Philadelphia.

Dam was designated for the job because Secretary of State George P. Shultz had planned to be on a trip to El Salvador and Brazil, a journey postponed early yesterday following the bombings in Beirut.

Shultz's postponement of a visit to the Central American hot spot of El Salvador because of an even more urgent eruption in Lebanon symbolizes the strain on U.S. policy in a period of unusually severe global and regional tension.

In the recent past, a U.S. flotilla was redirected from Central America toward the Persian Gulf as the long-running war between Iran and Iraq flared with new threats to the global oil lifeline.

A U.S. Marine amphibious force, which had backed up Marines in Beirut from offshore, left the Mediterranean after the Sept. 26 Lebanese cease-fire, lingering to show the flag near the Persian Gulf on the way home to Asian waters.

Last week, still another U.S. naval task force heading for Lebanon was diverted to the Caribbean when a coup in leftist Grenada endangered American residents and U.S. interests. The situation there is uncertain, but there were reports yesterday that the U.S. ships are resuming their passage to Beirut.

In addition to Lebanon, the Persian Gulf and Grenada, crisis spots on the government watch list include:

* Nicaragua, where CIA-supported guerrilla forces have stepped up attacks on petroleum facilities and the leftist Sandinista government has called for outside military assistance.

* Korea, where north-south tensions remain high following the Oct. 9 terrorist explosion that killed the top rank of the South Korean Cabinet in Rangoon, Burma. South Korea has blamed the bombing on North Korea.

* The Philippines, site of important U.S. military bases, where the rule of President Ferdinand E. Marcos continues to be imperiled by public and political reaction to the assassination Aug. 21 of opposition leader Benigno S. Aquino Jr. Marcos' handling of the assassination probe and growing economic trouble have deepened U.S. concern.

Meanwhile, demonstrations continue in Western Europe over the scheduled deployment of U.S. nuclear missiles in Germany and Italy by the end of the year.

As the backdrop for all of this, relations between the United States and Soviet Union have sunk to the lowest point in more than a decade following reactions in the two capitals to the Soviet downing Sept. 1 of a Korean Air Lines passenger jet.

The unusual degree of tension between Washington and Moscow has generated a poisonous atmosphere of superpower hostility surrounding a crisis-ridden world.