The congressional fact-finding mission is home from Grenada. Rep. Michael Barnes, among others, has concluded that the invasion of that island was justified. Rep. Don Bonker, among others, says it was not. Doubtless the argument will rage on inconclusively. But to focus entirely on justification may be to miss the broader point: whether the President did or did not have sufficient cause to set aside international norms and invade another nation, the invasions hardly represented a foreign policy victory for the United States.On the contrary, more than anything else Grenada was a monument to the mismanagement of foreign policy. It pointed up again the Reagan administration's incomprehension of the uses of diplomacy. For at least two years the Bishop government had been signaling its interest in reaching accommodation with the US: Bishop had pleaded for talks. Had the administration begun a negotiating process aimed at bringing our own influence and economic leverage to bear, the situation might have developed very differently. But the administration rebuffed Bishop. It had no more interest in a diplomatic process with the Grenadians than it has in one with the Nicaraguans and Cubans. One can only hope that the administration does not now conclude it can handle Nicaragua, Cuba, and El Salvador the same way it took care of Grenada. Landing marines worked in Grenada , a tiny island defended by a handful of Cubans and demoralized Grenadian soldiers. It has not worked and will not work in Lebanon. Nor will it in Central America. Yet, that is the direction in which the administration seems to be moving.And even were one to grant, for the sake of argument, that the invasion itself was politically defensible, there is no defense at all for the way the administration went about it. It had plenty of time to consult with our allies, yet did not do so, with predictable results. Our NATO allies were furious. Their confidence in our leadership and our sensitivity to their needs plummeted. The NATO alliance was needlessly damaged. Why? Can this be the same Ronald Reagan who criticized the Carter administration for failing to maintain the confidence of, and strong ties with, the NATO countries? Can this be the same Ronald Reagan who said good relations with Mexico and Canada would be the foundation stone of his policies for this hemisphere?One cannot but contrast Kennedy's effective diplomacy during the 1962 missile crisis with the lack of it during the Grenada invasion. In the first instance, there were careful consultations with our allies. They supported us to the hilt. In the second, consultations were only an afterthought. Our allies did not support us. Doubtless the two situations differed considerably in substance, but style, or lack of it, also played its role.Nor does it bother the President that we have been isolated and condemned in the UN and that world public opinion was swinging against us at a crucial moment - just before our deployment of missiles in Europe.Turning to the invasion's domestic implications, one must ask why American troops were sent into combat without proper consultations with the Congress. Whether in letter or in spirit, the Constitution was simply ignored. Those who believed the war powers act would restrain the President from committing troops in Central America should now know better.Finally, what about freedom of the press and the right of the American people to be informed? Newsmen were denied access long after there were any security grounds for doing so. We were back to depending on the official version of events, and, as in Vietnam, this proved unreliable. Military briefers, as well as those in the White House and State Department, gave out statements that were contradictory, inaccurate, and in some cases deliberately misleading. The day before the invasion, for example, one spokesman said the Americans on the island were in no danger and that there were no plans to remove them. A day later, the story was the exact opposite. ''Thousands of Cuban military personnel'' turned out to be fewer than 200. First there was a mass grave, then there was not. And it now turns out that the light Grenadian casualties first mentioned by briefers may not have been light at all. We will never be sure. The Pentagon's statements have been so evasive and contradictory that no figure it puts out now is likely to be believed.The President set the tone in all this. First he said it was an invasion, then insisted it was not, that it was a ''rescue operation.'' But in a rescue operation, our troops would have gone in, rescued our citizens, and departed. They did no such thing. They deposed the government, and we are now told they may be on the island for months. No rescue mission this, as the President well knows, but he doesn't mind calling an apple an orange.The image that emerges from the handling of the Grenada crisis, in sum, is one of an administration that was perfectly willing to set aside US law as well as international norms, which ignored the Congress, failed to consult with our allies, and, worst of all, had no qualms about misinforming the American people. It was not a reassuring performance.