President Reagan's latest decisions about the use of U.S. land, sea and air power in Lebanon have raised the stakes and risks for the United States in the Middle East, and have brought U.S. forces closer than they have been in many years to a combat role on the mainland of that strategic area.

How the United States has become increasingly engaged, step by step, over a period of 13 months is difficult to recall or understand. At nearly every step U.S. action was described as a response to inescapable demands or, as was the case in administration statements this week, as the logical extension of decisions previously made and announced.

Despite its incremental nature, the development of U.S. policy in Lebanon since the Israeli invasion in the summer of 1982 has created a large-scale American commitment to the government of President Amin Gemayel, which is now being pressed militarily as well as politically by a variety of forces, internal and external. The U.S. Marines in Beirut are a symbol as well as the flesh-and-blood reality of that commitment. And as the Marines in recent weeks have come under attack in an increasingly chaotic situation, their role has been expanding and the U.S. stakes involved have been expanding with it.

After the Israeli invasion of Lebanon, the United States sent emissaries, Philip C. Habib and Morris Draper, to stave off the destruction of Beirut by helping to arrange the peaceful withdrawal of Palestine Liberation Organization military forces.

As part of this arrangement, U.S. Marines as well as French and Italian troops landed in Beirut and supervised the evacuation of PLO fighters. After 16 days the Marines departed and the Europeans followed suit.

One year ago yesterday, on Sept. 14, a powerful bomb at his political headquarters killed Lebanon's President-elect Bashir Gemayel, setting in motion a surge of events that brought the Marines and the others back. One of those events was the Israeli occupation of west Beirut in the wake of Gemayel's death. Another was the massacre of Palestinian refugees in the camps on the outskirts of Beirut, a slaughter that shocked the world.

In sending the Marines back to Beirut at Lebanese government request, Reagan reported to Congress that they would be needed "only for a limited period to meet the urgent requirements posed by the current situation." He said "there is no intention or expectation that the U.S. armed forces will become involved in hostilities," that the U.S. agreement with Lebanon "expressly rules out any combat responsibilities for the U.S. forces" and that all armed groups in the area had agreed not to interfere with the multinational peace-keeping force.

The basic objectives of the U.S. military presence and that of the European partners were ambiguous. One objective, long since accomplished, was to bring about the withdrawal of Israeli troops from Beirut.

Another objective, cited in the U.S. agreement with the Lebanese government last Sept. 25, was "to establish an environment which will permit the Lebanese armed forces to carry out their responsibilities in the Beirut area."

A broader, more ambitious purpose was to enhance the possibility that Lebanon's travail could be used to create a strong government capable of cementing control of that divided and strife-torn country.

For months there was little interference or opposition to the 1,200 U.S. Marines and their European counterparts. As international negotiations to obtain the full withdrawal of Israel and Syria bogged down this summer, the internal tensions increased and clashes began among armed elements of religious and tribal groups, including Maronite Christians, Shiite Moslems and Druze. Full-scale battles erupted when Israel began a partial pullback from hotly contested areas.

Two U.S. Marines were killed by artillery and mortar fire Aug. 29, and two more U.S. Marines under a similar barrage Sept. 6. When Marines were attacked by ground forces, they returned fire with small arms. When attacked by artillery, they returned fire with their own artillery and called in helicopter gunships to attack those firing at them.

To protect the 1,200 Marines and their mission, Reagan on Sept. 1 ordered a naval task force of 2,000 more Marines and heavy artillery and fighter planes to the nearby Mediterranean within easy reach of Beirut. The aircraft carrier USS Dwight D. Eisenhower, already in the area, was moved closer to Beirut. The French, who also had suffered casualties, brought up their own aircraft carrier and accompanying warships.

On Sept. 7, U.S. and French warplanes flew over Beirut for the first time in a show of force as battles continued. On Sept. 8, U.S. naval guns offshore for the first time fired back to silence guns that attacked the Marines from nearby mountains.

In a still-deteriorating situation, the White House announced Tuesday that the Marines have been empowered to call for air strikes from the fleet as well as ship artillery to protect themselves.

Moreover, for the first time the White House extended the concept of "self-defense" of the Marines to the fortunes of the Lebanese army, saying that U.S. airpower and artillery might be used if Lebanese armed forces were attacked in positions of strategic importance for the eventual defense of the Marines. Officials said they had in mind the key mountain village of Suq al Gharb, but their statements were not limited to that area.

Thus U.S. military forces originally introduced for political support and peace-keeping missions in a non-combat role fought back when attacked and then were reinforced by extra air and sea power when the attacks continued. Now U.S. military power has been displayed, used and threatened in battles within Lebanon, and is in position to become further involved if trouble continues.

In recent days the military presence also has taken on a deterrent role, symbolizing potentially powerful U.S. military intervention in case of opposition, especially from Syria.

The challenges posed for the United States are at several levels:

At the local level, the U.S. military and political actions add up to a large-scale commitment to the increasingly-embattled Gemayel regime. It is unclear that the presence of U.S. military force can turn the tide for the Lebanese government in the absence of a growing U.S. combat role against various Moslem elements, but it seems clear that withdrawal of the Marines now would bring about the government's collapse. A negotiated political settlement among rival groups within Lebanon is the main hope for averting a deteriorating situation.

At the regional level, Syria's political, logistical and possibly direct military involvement with the local foes of the Lebanese government is bringing Washington into conflict with Damascus, which has exercised a crucial role in Lebanon.

Reagan described the offshore U.S. naval task force as "a marker for Syria." Israel's stakes, its partial disengagement from Lebanon and its reluctance to use its power for U.S. and Lebanese government purposes, are complicating factors. How the United States deals with Lebanon, Syria and Israel politically and militarily is likely to have major impact on such Arab allies as Saudi Arabia and Egypt.

At the global level, the Soviet Union's increasingly close alliance with Syria, including the presence of more than 5,000 Soviet air defense and other military personnel in Syria, adds a superpower military dimension at a time when U.S.-Soviet relations are gravely strained for other reasons.

U.S. decisions regarding Lebanon also will affect Washington's relations with France, Italy and Britain, who have their own commitments and troops at risk in an increasingly risky situation.