THE INF TREATY drawn by Moscow and Washington changes a part of the nuclear equation and a part of the political equation, and so it was time for the Atlantic Alliance to get its policy together for the next East-West phase. This was the purpose of the NATO summit in Brussels, the first in six years, and it did the job passably well.

There are those who believe that the INF treaty tilts the nuclear balance against the West by ''decoupling'' Europe from sure American nuclear patronage. Even people who don't believe it recognize a political need to steady European nerves on this score. Especially is this so in the face of Mikhail Gorbachev's post-INF campaign to eliminate a third category of Europe-based nuclear weapons, the short-range and battlefield sort that the INF treaty does not limit and that NATO has long agreed to modernize. The key here is West Germany, principal host for these weapons and a country split on whether to eliminate them or modernize them. In Brussels, this issue was addressed in NATO's time-sanctioned way of compromise. The Germans were held to the alliance commitment to modernize but given some flexibility in accomplishing it.

That left the larger question of meeting the Gorbachev ''peace offensive'' aimed at loosening Europeans' ties with Washington and each other. Necessarily, one large item of that strategy rests with the alliance leader and its talks on strategic arms. Another large item rests with alliance members and talks they are trying to organize on conventional arms -- a project that the INF Treaty, jostling allied confidence as it did, made urgent. In Brussels, NATO gave a useful boost to a strategy based on cutting the tanks and artillery that give the Soviet Union, alone, the possible option of a surprise attack. There are miles to go before talks open, let alone before they succeed, but the summit points a way.

The alliance constantly disappoints people who like things neat and orderly, but for almost 40 years, including the Reagan years, it has bound the United States in a relationship that has given Western Europe unprecedented security and prosperity and offered Eastern Europe a glimmer of another path. These days, it is rolling with the INF Treaty and its difficult aftermath. France, long the odd man out, is moving partly back in, and rivals Greece and Turkey are starting, just starting, to act as if they were allies. NATO is strained but striving as it moves into a stage of East-West relations marked more by sophisticated political competition than raw physical threat.