Lebanon revives classic query: Who makes foreign policy?

President Reagan's proposal to send ''a small contingent'' of US marines into Lebanon to help extricate trapped PLO forces is received dubiously in Congress. Who makes foreign policy in the United States - the White House or the Legislature? The Constitution is ambiguous.

The prospect is that in this emergency once more the answer will turn on events rather than on a formalized policy between White House and Congress.

The question is being tackled as the Senate confirms a new secretary of state , as the administration's foreign-policy making apparatus is in disarray, and as Lebanon is in flames. Historians shrug. This isn't the first time that the United States has implemented diplomatic initiatives by extemporization rather than by calculation.

It started with a leak. President Reagan announced the plan at Los Angeles, July 6, after the Israeli press had divulged his secret approval of use of the marines. The proposal was to land about 1,000 at Beirut for a 30-day stay to help evacuate Palestine Liberation Organization fighters.

Since then the crisis has deepened; past memories of ''sending in the marines'' have revived, and deeper ambiguity has settled over the whole situation.

Some analysts think President Reagan will modify his proposal; others wait to see if a classic case of disagreement is developing over foreign policy. Noted scholar Edwin Corwin argued that the Constitution is an invitation to President and to Congress to struggle over who makes foreign policy. The President is commander in chief, rules the executive, controls information and seems to have unassailable preeminence in foreign affairs. On the other hand, most foreign-policy powers enumerated in the Constitution are with Congress, and it is instructed to provide for the common defense, to control the purse, and, among other things, to declare war. In theory, at any rate, President and Congress are partners.

Sixty years ago Congress rejected Woodrow Wilson's League of Nations, opening the way to World War II 20 years later.A chronology of more recent events:

* The 1972 War Powers Resolution, passed over a Nixon veto, imposed procedural restraints on the unilateral power of the president to commit combat troops abroad, requiring congressional authority within 60 days.

* The Jackson-Vanik amendment to the Trade Act of 1974 tied detente to freedom of emigration from the Soviet bloc, particularly for Soviet Jews.

* Congress in 1974 and 1976 increased oversight over CIA covert activities.

* Congress put strict conditions in 1978 on export of nuclear equipment.

* Senate amendments to the Panama Canal Treaties of 1977 almost forced renegotiation.

* SALT II was blocked.

* The Taiwan Relations Act of 1979 nearly derailed rapprochement with mainland China.

In response to the above the White House has sidestepped treaties and relied more and more on ''executive agreements'' (which avoid Senate ratification.)

In summary, Congress is going through an assertive period of foreign policy control. It could find dramatic illustration in the new Lebanon crisis.