The assassination of Sen. Benigno S. Aquino just as he returned from exile to the Philippines will have important consequences for the Philippines and the United States. It is not too soon to say that it is one of those moments in history when the future is determined. How the US re-acts may well decide if civil war can be averted.
His death is important because of what it says about the state of affairs in Philippine ruling circles, the clear signal it sends about democracy to the Filipino people, and the difficult choice it presents the Reagan administration. It appears as an act of desperation but is probably indicative of a succession struggle within the government.
The Marcos government reacted with an unusual sense of apprehension to Aquino's return: his passport had expired, he could not be issued a new one, he would be sent back immediately on the same plane, any airline which flew him would be fined, his safety could not be guaranteed, assassins awaited. Why this concern for a man many Filipinos believed had been discredited by too long an exile in America and possibly covert dealings with the very regime that had imprisoned him?
It was because, after more than a decade of one-man rule in the Philippines, Aquino still represented the only viable alternative to Marcos. In March 1978 he had been permitted to campaign from prison for the opposition Laban Party and almost led that party to victory. His was the most charismatic alternative to Marcos. Fifteen years younger than Marcos, he could afford to bide his time, organize his party, and run in the 1987 presidential elections with a fair degree of certainty of winning even if he played by Mar-cos's rules. He was a serious challenge not only to Marcos but to anyone who wanted to succeed Marcos.
But what is important is not that Marcos or someone in his government may have ordered Aquino's death but the perception by all that only they could have done so. Thus, those in the past who opposed Marcos but did not support Aquino will see in his martyrdom that the only alternative to Marcos's rule is revolution. It sends a clear signal to Filipino voters that the 1984 General Assembly elections and the 1987 presidential election will be a sham.
As in Central America, the moderate middle will be driven to extremes. It will further radicalize the middle class - members of which were responsible for the most recent bombings in Manila. The country's economy, already in grave difficulty, will worsen as foreign investors uncertain as to political stability turn away. And it suggests that the succession struggle is aleady underway within the ruling clique.
The Reagan administration is caught in a severe quandary. Secretary Shultz visited Manila in June at which time he praised the country's move toward democracy. Vice-president Bush's infamous praise during his 1981 visit (''We love your adherence to democratic principles and to the democratic process'') still rings. President Reagan has plans to visit the Philippines this November. The White House would be well advised to reconsider.
For President Reagan not to go is to suggest that the US believes the Philippine government is responsible for Aquino's assassination and/or believes the country is so unstable that Mr. Reagan's security cannot be guaranteed. That suggestion could well promote the collapse of Marcos's regime.
For Mr. Reagan to go is to send another signal to the Filipino people: The US will back Marcos no matter what his government does. In 1991 the US basing agreement expires. If Marcos is not in power then, the US may well find that its identification with him has cost it a major element of its Pacific defense strategy.
This, however, may be the last opportunity for the US to show its support for democracy and moderate change in the Philippines.
During the Shultz visit, his aides reported that the US was trying to open ties to opposition leaders as the regime was in its ''twilight.'' Now there is one less opposition leader for the US to talk with and a regime moving beyond twilight into night.
Richard J. Kessler, a visiting research fellow at the University of the Philippines 1978-79, is a fellow at the Center for Strategic and International Studies, Georgetown University.