If the first casualty of war is truth, a subsequent casualty is often the Constitution. So it is with the Reagan administration's Central America war. The war is propelling a return of the ''imperial presidency,'' an executive branch that operates unchecked and beyond the bounds envisioned by the Founding Fathers.
President Reagan's actions in Central America have undermined three major congressional checks on the warmaking proclivities of recent presidents: the War Powers Resolution, the power of the purse, and legislative oversight.
Passage of the War Powers Resolution was animated by unilateral presidential decisions to invade the Dominican Republic and to involve the United States deeply in the Vietnam war.
Today, without congressional approval, US pilots provide ''real time'' reconnaissance for Salvadorean troops and direct them to guerrilla positions. US troops have already come under hostile fire. The US Navy patrols coastal waters all along the Central American isthmus. US military exercises in Honduras have placed the region on a war footing.
These are precisely the sorts of unilateral executive commitments that the War Powers Resolution was supposed to prevent.
Congress has few means to enforce the law. This year the Supreme Court dismissed a suit by 29 members of Congress who alleged that the presence of US troops in El Salvador violated the War Powers Resolution. Earlier the court had overturned the section of the law that would have enabled the Congress to recall soldiers from a conflict before a 60-day period had expired.
The legislative branch could try to restrain the President through its power of the purse. But Congress has been reduced to bickering over presidential requests for military assistance to El Salvador and Guatemala. The major commitments have been undertaken outside the appropriations process, and even much of the military aid evaded direct congressional authorization.
President Reagan, moreover, has intimated he will defy appropriations laws that might restrain him. He has supported the Central Intelligence Agency and the Defense Department against charges that they have spent money illegally.
The House Intelligence Committee recently alleged that the CIA overspent its legal limits in the covert war against Nicaragua by borrowing Defense Department equipment such as a mother ship. The agency blithely claims that this is a mere disagreement over accounting procedures.
Similarly, the General Accounting Office has found that the Defense Department violated an appropriations law in building permanent communications stations, Army barracks, and improved air bases in Honduras. The department arrogantly responded that the facilities are necessary for the temporary exercises there, which are scheduled to continue until 1988.
Such arrogance not only makes a mockery of the appropriations process, it undermines the important legislative responsibility for oversight of the executive branch.
Oversight requires full access to accurate information so that the Congress could stop actions when they were inconsistent with the law.
Yet the executive has repeatedly tried to keep the legislature in the dark. Mumbled and obscured disclosure of CIA plans to mine Nicaragua's ports in violation of international law, for example, prevented the Congress from acting as an effective check on the President. It was public embarrassment and demands from US allies that prompted President Reagan to halt the mining.
The dangers of an unchecked presidency were obvious to the framers of the Constitution. The war in Vietnam, along with unrestrained CIA activities, made them obvious to Congress a decade ago. Today's war in Central America has not yet ballooned the executive's powers to the size of an imperial presidency. But Congress must act now to assert its powers, before the war claims the constitutional order as its victim.