Those of us who have argued that systematic terrorism is a form of warfare, to which the United States must respond, naturally find the retaliatory strike on Libya a necessary and proper step. President Reagan's rationale is ours: unless we are prepared to live under perma- nent siege, there can be no safe haven given those who harbor and sponsor terrorists.

In that respect, the casualties suffered in the raid on Libya are comparable to those the FBI suffered in the Miami shootout with the suspected bank robbers. The perpetrators of international crimes -- and terrorism is a particularly heinous one -- must be hunted down and pursued as vigorously as domestic lawbreakers.

But even as the vast majority of Americans endorse the president's action, it is important that we not deceive ourselves about the difficulties and dangers in the course he has chosen.

At least three of them became visible in the days immediately following the aerial assault on Muammar Qaddafi's headquarters. This was "a meassponse" designed -- though unsuccessfully -- to minimize civilian casualties and property damage. From all accounts, Reagan has been insistent that every possible step be taken to spare the innocent.

One can respect that sentiment and still point out that such a limited retaliatory step leaves the criminal free to strike again. Unless it has the effect (as some administration officials hope) of weakening Qaddafi to the point that his domestic opponents can overthrow him, there is every reason to think that Qaddafi's terrorists will lash back at us.

Whatever its effect on Qaddafi, a protracted series of tit-for-tat, terrorist- counterterrorist exchanges will leave the citizens of this country demoalized and frustrated. It will increase the hazards to Americans or restrict our freedom of movement, or both. What is now applauded as a show of strength will -- if often repeated -- begin to appear as ineffectual and self-mocking as the "yellow ribbon" response to the Iranian hostage ordeal.

The second risk is that our handling of this challenge will inevitably affect other, even more important, issues. The reluctance of all of our European allies ex cept Britain to support our action did not -- and should not -- deter us. But the campaign against terrorism is being fought closer to their cities than to ours, and over time this division can become troublesome.

It can weaken the confidence of their citizens and governments in the judgment of America's leaders on a critical issue of international security. And, even more to the point, it can erode support in this country for the expensive commitment we have made to the defense of Western Europe -- the anchor of the policy that has kept peace between the great powers for more than 40 years.

The action against Qaddafi has already intruded on Soviet-American relations. The Russians' decision to postpone the foreign ministers' scheduled talks jeopardizes the second Reagan summit with Mikhail Gorbachev. But in itself it is a minimal gesture of support for a Soviet client and does not make it impossible for the two leaders to meet later than the originally desired summer session.

But once again, a protracted series of Libyan-American terrorist-military exchanges can become an obstacle to great- power talks dealing with larger regional issues and the overriding problem of nuclear arms control.

The third risk is that subduing Qaddafi can become the preoccupation -- the consuming passion -- of American foreign policy, to the detriment of other, larger interests. Reagan has displayed a weakness for confusing vultures and gnats. His focus on the weak, impoverished and tiny country of Nicaragua has diverted our attention from the challenges and problems in the much more consequential nations of Mexico, Argentina and Brazil.

State-sponsored terrorism is not confined to Libya; Syria and Iran are far larger sources of programmed violence. And the force that feeds extremism in the Arab world -- the unsettled Palestinian problem -- cannot be addressed through a bombsight.

All of these matters suggest that a limited counterstrike against Qaddafi's terrorism, while fully justified, is more effective as a signal than as a long-term policy. Indeed, if the same kind of action becomes necessary again and again, it will inevitably become more costly to our interests.

The deadly logic of the situation implies that we are embarked on a course of inevitable, and perhaps drastic, es- calation, unless Qaddafi's domestic enemies take the situation into their own hands. Americans can take justifiable pride in the president's rejection of passivity in the face of deliberate provocation. But the path ahead will not be easy or painless.