Well, ''it'' finally happened this week - or, to be precise, it started to happen. ''It'' in this case is the most controversial military move in East-West relations since the decision in 1950 to set up a combined and unified NATO military force in Europe with an American in command.
''It'' was the actual landing on an English airfield of the first American intermediate-range cruise missiles designed to reverse the trend of recent years toward potential Soviet nuclear superiority in the European theater.
Quietly, last Monday, United States transport planes touched down at the Royal Air Force's Greenham Common base and began unloading the first of 16 cruise missiles which have been scheduled for the base by NATO planners since 1979.
Not one is yet operational. That step will come in the second part of December. By then, presumably, the first of a second lot of 16 will be arriving in Italy and the first of another lot of 9 Pershing II ballistic missiles will be arriving in West Germany.
The 41 new weapons in this first round of deployment will be followed by more during the next four years. The plan calls for a total of 572 of the new weapons to be in Europe by the end of 1988 - unless in the meantime there has been an agreement with the Soviet Union on the level of weapons in the European theater.
The military significance in this week's deployment is in the range of the new weapons. Even when all 572 are in place, if they ever are, the total number of nuclear warheads in NATO hands will not be increased. NATO has announced it is beginning to scrap about 2,000 of its older-model weapons. The real difference is in the range and accuracy, not in the number, of the new weapons.
The Pershing IIs in West Germany and the cruise missiles in England and Italy will be able to reach well into the Soviet Union. They can cover an arc that extends, roughly, from the mouth of the Dnieper River to Leningrad. They would not quite reach Moscow. But they could reach Soviet military depots, bases, and command centers within the arc short of Moscow. Odessa, Kiev, and Minsk are inside the arc.
The move into the longer-range weapons for Europe started with deployment by the Soviets of their SS-20 weapons. Previously, just as did the NATO countries, they had shorter-range weapons with which they could launch nuclear warheads against any target in Western Europe. But the SS-20 more than doubled the Soviets' range, meaning that from well inside the Soviet Union they could reach NATO command posts and launching sites while their own were out of reach of NATO's intermediate-range weapons.
The intensity of the Soviet propaganda campaign against this week's initial NATO deployment is presumably explained partly by the fact that the new American weapons will be able to reach into Soviet areas that had previously been covered only by longer-range strategic Western weapons.
Thus, as the new American weapons come on line and become operational, the Soviets will have weighted against them an increase of up to 572 in the number of warheads that can be lobbed inside their frontiers.
There is a second reason for the vehemence of the Soviet propaganda campaign against the new NATO deployment. It has been useful to them in their major and long-term efforts to sow dissent within the NATO alliance. They have twinned the idea of new American weapons in Europe, Americans at the controls, with the image they have been building of a trigger-happy American President. The invasion of Grenada and more recent talk of US ''reprisals'' against Syria have been played up by the opposition in Europe to reinforce this ''trigger happy'' image.
Some damage has undoubtedly been done to the alliance in pushing ahead with the deployment. Whether the strategic gain outweighs the damage is open to question. The answer is in the future when the alliance is next tested.
Meanwhile, the more interesting question is what the Soviets will do in response. That they will do something is obvious. Apparently, they will not break off arms control negotiations. At least, their negotiating team at Geneva announced after the news of the first deployment that they would be back for more talks.
It is more likely they will take a next step in weapons deployment themselves. They could do to the US what the cruise and Pershing II deployment does to them - increase the number of warheads aimed at US territory.
They could mount some of their SS-20 weapons in northern Russia or Siberia. The SS-20 has a range of 3,000 miles. Also, they have their own version of a cruise missile. It is the SS-C-1b, known in the West as Sepal. It has a range of about 300 miles. It could be mounted in submarines which could then cruise off American coasts.
A reasonable guess is that they will not proceed toward serious negotiating on these new intermediate-range weapons until they themselves have done whatever they intend to do to balance off the Pershing II and cruise missiles in the new Western arsenals.
The reasonable prospect therefore is that new NATO deployments will continue for some time - accompanied of course by all the antiweapons demonstrations that the peace movements of Europe can manage.