They could have held a war last night and nobody would have gone. That's because the ambassadors of just about every country, along with Secretary of State Alexander Haig, were otherwise engaged at the Northwest home of international lawyer and embassy confidant Sylvan Marshall, who held a command performance honoring Sen. Charles H. Percy (R-Ill.), the new chairman of the Senate Foreign Relations Committee.

Superpowers couldn't help rubbing against thorns in their sides in the crowded passageways. When asked about Poland, Soviet Ambassador Anatoliy Dobrynin conveniently deferred to his satellite, who was trying to mingle somewhere near the sunken rock and gem garden.

"Ask the Polish ambassador, he's right over there," Dobrynin said, ducking into the cloakroom.

"Outside, an irate neighbor leaned on his horn in the face of a black limousine gridlock. Inside, Marshall watched the door. Finally, he boomed out over the heads of the crowd, "The secretary is on his way."

Just about everybody there acted as if they wanted to get Haig's ear, but he didn't seem to feel like anything but party chatter. Haig came late, left early and kind of brushed right by Percy and his wife, Loraine.

One of Haig's current sparring partners, President Reagan's budget whiz David Stockman, did not attend. But Haig dealt with questions about the disagreement between him and Stockman with official magnanimity.

"We're all going to get our way," Haig said. "It's a combined team."

Meanwhile, the Percys dutifully kept sentry in the drafty foyer. From Saudi Arabian Ambassador Faisal Alhegelan they got smacks on both cheeks and exuberant hugs, as they did from former dean of the diplomatic corps Guillermo Sevilla-Sacasa. Chilean Ambassador Jose Miguel Barros was a bit more restrained.

Percy, who has taken some flak for his talks with Soviet officials during a recent trip to Russia, spoke of the dialogue he hopes to perpetuate between the White House and his committee room on issues like aid to El Salvador, relations with Iran and possible cuts in the foreign aid budget.

"The Foreign Relations Committee would have to work with the adminstration for not only a bipartisan policy but a nonpartisan policy," Percy said. "We always reserve the right to have different views, but we should be constantly consulting back and forth so we speak with one voice. It's hard to speak with one voice when you have Haig and Stockman, who have different views. Among our allies, there's confusion about whay our policy really is, and on the part of our adversaries."

Reagan counselor Edwin Meese III briefly held court in the back room, near a shelf adorned by a silver trinket inscribed in 1978 to "Mara and Sylvan" from former Iranian ambassador Ardeshir Zahedi.

Meese claimed to put no stock in recent reports that Cabinet meetings are being reorganized to prevent leaky contradictions. In response to a question about what is being done to change the sessions, he replied, "Nothing."

"We have a whole lot of different Cabinet meetings," Meese said. "We have full work sessions, work sessions with and without the president."

Those who stood in line for beef shishkabobs, tiny reubens, artichoke hearts and other party circuit delicacies included Sen. Harry F. Byrd (Ind.-Va.), former senator Hugh Scott, Rep. Sonny Montgomery (D-Miss.), Rep. Charles E. Bennett (D-Fla.), Deena Clark and former American University chancellor Joseph Sisco.

When asked how a man who is relatively invisible on the public scene could produce such a showing of the discreet power elite, Marshall tried for some of the cool, diplomatic evasiveness for which his embassy clients are known.

"They know I'm a good cook," he said.