In more than 2 1/2 years in the White House, President Reagan has relied on theatrical demonstrations of U.S. military power to try to shape events in trouble spots all around the world.

He has sent sophisticated airborne warning and control system (AWACS) planes to the Middle East, Africa and the Caribbean.

He has dispatched aircraft carriers as shows of force off the coasts of Libya, Central America and now Lebanon.

He has ordered military exercises as demonstrations of U.S. resolve in the Middle East and Central America.

What his administration calls "military trainers"--rather than the Vietnam-era term "military advisers"--have been sent to train beleaguered troops of numerous U.S.-supported governments, including those of El Salvador, Honduras and Lebanon.

All this military activity, while dramatic, has not been very risky, either physically for the men involved or politically for Reagan. Voters understand that American men in uniform are paid to take certain risks. The United States now has an all-volunteer Army, which is politically invisible compared with the draft, which stirred up protests throughout the Vietnam war.

However, Reagan could be confronted with going a risky step further in Lebanon if the Lebanese army is unable to quell warring factional militias without the active assistance of the multinational peace-keeping force of American, French, Italian and British troops there.

If the U.S. Marines in that force were to be ordered to switch from static self-defense to a more active role in restoring peace in Lebanon, they would be sure to suffer significant casualties. Infantry probing, especially in Lebanese cities, where fire can come from any place or any sect, is a high-casualty business, as the Israeli army has found there.

One administration official said yesterday that the military and political risks, plus the Israeli experience, are all being considered as the administration tries to decide what to do next militarily in Lebanon.

He predicted that there will be no immediate change because the risks of doing something more with American military power than merely displaying it are too great. Instead, the administration is still hoping the Lebanese army can restore order.

"If we leave, we're showing the white feather," said one Marine general this week. "If we stay as we are, we have to sit and take it without doing much for the Lebanese army. If we go out with them on sweeps, we've changed our mission and gone into combat."