A plank has two ends

WHEN I was a boy my father watched me trying to carry a large plank through the house without doing damage to the furniture. He had occasion to say: ''Remember, that plank has two ends.''

While watching the Reagan White House try to arrange the future of Lebanon and now the future of Central America I have felt the urge to repeat my father's injunction. They are watching the front end of what they are doing, but seem too often unaware of the back-end consequences of their front-end actions.

The failure to think through those back-end consequences came out clearly in the Lebanon affair. They first backed the leadership of the Maronite Christian minority. Then they arranged to have that leadership negotiate an arrangement with Israel which was manifestly unacceptable to the Muslim majority and to the outside country with the most immediate and major interest in Lebanon - Syria.

The operation might have succeeded if the Soviet Union had failed to see the opportunity opened up for Moscow. But the Soviets seldom overlook such opportunities. They proceeded to resupply Syria with enough modern weaponry to make it secure against an Israeli attack. Then all Moscow had to do to spoil the operation was to sit back and let nature take its course.

Once the Syrians could defend themselves, they were bound to object to a deal that would have displaced their own primary influence in Lebanon and put Israel in their place. The deal sponsored by the White House would have given Israel both the political and economic inside position in a Beirut run by the Maronites whose militia has been in the pay of Israel.

Syria, when backed effectively by Moscow, could spoil the proposed deal. It did. Washington's experts on the Middle East had warned of what would happen. Joseph Sisco, who heads the list of those experts, was formerly the State Department officer in charge of the Middle East. He told them from the beginning of the operation that they should negotiate with Syria first. They did not listen.

Now the White House is engaged in an operation aimed avowedly at forcing Nicaragua to cease and desist from supporting the rebels in El Salvador. The method employed is in two parts. Part 1 is to build substantial American military force in the area. Part 2 is to back emigre groups of disaffected Nicaraguans who would like to overthrow the government of their country in a counterrevolution.

This double operation is now well advanced. The amount of US force in neighboring Honduras is capable of launching a US invasion of Nicaragua. The potential of such an invasion backs the activities of the rebel forces. Washington is better prepared for an invasion of Nicaragua today than it was for the invasion of Cuba in the Bay of Pigs affair of 1962. But neither congressional nor public opinion is ready for any such American invasion.

The Bay of Pigs was a fiasco. All the Cuban emigres who were landed on Cuba ended up either killed or in Fidel Castro's prison camps. The net affect of the operation was to solidify the Castro regime at home and the regime's ties with Moscow.

In this case US public opinion gives little sign of eagerness for an invasion of Nicaragua. Major religious communities have openly criticized the military nature of the operation. These include Roman Catholics, Methodists, Lutherans, Presbyterians, and Episcopalians. Congress is unhappy. The risk is that public and congressional opinion will prevent the President from doing what he is preparing to do.

Such disapproval has already caused the White House to call off the mining of Nicaraguan harbors. Perhaps the ''contras'' will continue to get funds from the CIA. But it is unlikely that US troops will invade Nicaragua. Without such support the chances are that, as at the Bay of Pigs, the counterrevolutionaries will discover that they have few friends inside the country.

In other words the White House has launched another operation which it will not be allowed by Congress or public opinion to finish. In world affairs if you start and fail to finish, you are usually worse off than had you never started.

The problem is not that the purpose is wrong, but that the methods used are not good enough to win.