The United States is now involved in applying military pressure on Syria for a political purpose. The political purpose is to convince the Syrians that they cannot have things their own way in Lebanon.
The use of military power for a political purpose works (usually) when the military advantage is decisively on the side of the person using the power.
Grenada was a spectacular case in point. The US enjoys military superiority in the entire Caribbean area, in virtually all the Atlantic Ocean. Grenada is deep inside Washington's potential military frontier. The US could (and did) do what it likes inside that frontier.
Nicaragua is probably going to be another case in point. The indirect application of US military power, through CIA support of Nicaraguan rebels, has caused the Nicaraguan government to modify its domestic policies and move toward political accommodation with Washington. It cannot ignore the potential power of the US in Central America.
The Middle East is not deep inside the US military frontier. It lies closer to the Soviet Union. At the nearest points, Syria, which the US is attempting to coerce, is less than 300 miles from the Soviet Union. The US is more than 5,000 miles away. US military strategists usually assume that in the event of war between the US and the USSR, the US would have to withdraw its naval and air forces from the eastern Mediterranean - perhaps the entire Mediterranean.
The US position in the Middle East is in some respects worse than the US position was in Vietnam. In Vietnam the US was fighting a Soviet client separated from the Soviet Union by both India and China. The Soviet supply line to the Vietnamese was a long one. Their supply line by air to Syria is only a few minutes. It is longer by sea through the Dardanelles, but far shorter than for the US across the entire Atlantic.
In another respect the US strategic position is as bad as in Vietnam. There the US supported a regime left over from French colonial days, thus alien to most local minds. The Soviets backed native Vietnamese nationalism. It was the winning advantage. Vietnam is still a willing client of the Soviet Union.
In the Middle East the US is associated (more than ever since last week's visit to Washington by Israeli Prime Minister Yitzhak Shamir) with Israel, which to any Arab and Muslim is an alien element in the Arabian Peninsula. The Soviet Union is the prime supplier of weapons to Syria and stands ready to help other Arab powers against Israel.
On Nov. 22, US Defense Secretary Caspar Weinberger said that the Oct. 23 truck bombing of US marines at the Beirut airport had been done under the ''sponsorship and knowledge and authority of the Syrian government.'' Secretary of State George Shultz and President Ronald Reagan have said that they hold Syria responsible for acts committed against American forces in Lebanon.
Under the doctrine of Syrian responsibility, the US has now engaged in an act of war against Syria. It has bombed Syrian military positions in Lebanon. (It is an interesting footnote here that the French and Israelis, who were also the victims of truck bombings, have refrained from holding Syria responsible and have not attacked Syrian positions.)
So we come to one essential fact. The US alone has applied its military pressure on Syria, which is a military client of the Soviet Union, in a part of the world where Soviet air power has a strong potential advantage over American air power and where the ultimate ability of the US to dominate the military situation is in doubt.
France and Israel have retaliated against Syrian proxies - Druze and Shiite Muslim irregular forces. Only the US has operated under the doctrine of Syrian responsibility.
The US has not yet applied its power against Syrian forces based inside Syria. But the doctrine of Syrian responsibility could lead to that. There are said to be about 7,000 Soviet troops in Syria helping to man Syrian positions along the Lebanese and Israeli frontiers.
The US could not lose when it invaded Grenada. It cannot lose against Nicaragua, provided it is willing to pay a higher price than in Grenada. But when it applies its military power on Syria, which is backed by the Soviet Union and can be supported and resupplied by the Soviets in minutes, we are all in a new and very different game, in which Moscow holds the trump cards.