THE UNITED STATES and other countries are being pushed toward consideration of a question most of them have preferred to evade. Who started the Gulf war? The reason for the evasion is that Iraq, the country most others are eager, for geopolitical reasons, to save from defeat at Iranian hands, is the principal guilty party. True, it was not black and white: Iran engaged in provocation and subversion. But it was dark gray and light gray: Iraq sents its troops over the border, strongly indicating, moreover, an intent to annex certain Iranian territory. This blunt fact inevitably embarrasses Western, Soviet, Arab and others' attempts to spare Iraq some of the consequences Iran has sought to inflict upon it: not just military defeat but Saddam Hussein's ouster, vast reparations and international condemnation.

It is the item of international condemnation that is at center stage now in the aftermath of the Gulf peace mission undertaken by the United Nations secretary general. Javier Perez de Cuellar went out to win Iran's and Iraq's consent to a unanimous Security Council appeal for a cease-fire and settlement. Iraq was ready if Iran was, but Iran hung back, putting its special emphasis on a provision in the U.N. resolution that would set up an impartial commission to rule on the origin of the war. The council inserted this provision to win back the diplomatic access to Iran that it abandoned early in the war when it ignored Iran's protests that Iraq had invaded.

Pronounce Iraq an aggressor? There are immense difficulties in the way of stigmatizing a country that, innocent or guilty, has become identified with the American interest in the stability of a vital region. Not stigmatizing Iraq may damage that interest, but doing it could damage that interest too. Yet the importance of the ''justice'' issue to Iran and Iran's capacity to continue pressing its counterinvasion of Iraq are political facts. As things are going now, the two combatants are resuming military action in an atmosphere of general pessimism about the diplomatic prospects. Increasingly it appears, nonetheless, that any effective search for alternatives to indefinite conflict must take into account Iran's extremely inconvenient but not unfounded complaint that Iraq began the war.