More than any other new American weapon, the Pershing II missile appears to rattle the Kremlin.

This new missile is at the heart of Soviet concern over the planned buildup of North Atlantic Treaty Organization missile strength in Western Europe. It also is probably the main reason behind Soviet President Leonid Brezhnev's threat yesterday to take "retaliatory steps" that would put the United States in an "analagous position" to the Soviet Union if these new NATO weapons are actually deployed.

What the mobile Pershing II missile will be able to do, if it works properly, is something that no other atomic weapon in the western arsenal can do now. Launched from sites in Western Europe, it will be able to fly 1,000 miles in six to eight minutes, and land with high accuracy and virtually no warning on targets deep inside the Soviet Union.

The United States has thousands of other nuclear-tipped missiles and atomic-bomb-carrying aircraft capable of hitting the U.S.S.R. But the big intercontinental-range missiles based in this country take some 30 minutes to reach their targets. Submarine-based missiles take only 15 minutes but are less accurate. Bombers take hours and can be shot down. Even new cruise missiles, which are supposed to be deployed in Western Europe along with the new Pershings, fly like small jet airplanes, take at least half an hour to reach their targets and can also be shot down.

Only the Pershings could hit the Soviets "out of the blue," in a surprise attack with essentially no warning. Thus the Soviets fear these weapons could be used in a first-strike against them that could quickly disrupt their military command centers and formations at the outset of a war and even strike their underground missile silos with sufficient accuracy to knock them out.

Although the Pershing II will carry only a single warhead, equivalent to the power of 200,000 tons of TNT, the Soviets believe it will eventually carry multiple warheads and will be able to fly much farther into the Soviet heartland.

The Soviet concern about the Pershing is not new. It has been spelled out in new Soviet pamphlets distributed in the West in the past year as it became more and more clear to Moscow that NATO was going ahead with its deployment plan.

While the Soviets also call the cruise missiles a first-strike weapon, because they are small and hard to detect and thus also could strike with little or no warning, it is clear that the Pershing II is the centerpiece of their concern.

American officials understand this and many months ago said privately that the Pershing II is the one reason why Moscow may eventually decide to negotiate seriously in the arms talks that opened in Geneva last November and that are aimed at reducing or removing the mutual threat in Europe.

The Pershing II is also at the center of the confusing math of these arms talks, in which the Soviets say there is now a kind of parity in weapons while the Reagan administration says the western alliance is greatly outgunned in Europe.

The NATO alliance plans to begin fielding 108 Pershing IIs and 464 cruise missiles in 1983 because Moscow already has some 300 new mobile SS20 multiple-warhead missiles deployed, with most of them targeted on Western Europe. The Soviet weapons could strike any NATO target in Western Europe within 10 to 20 minutes of launching from their bases in the western U.S.S.R..

Western strategists say the new U.S. weapons are essential to balance the striking power the Soviets have and are crucial to convincing them that they could not get away with a limited attack on Western Europe--as opposed to an all-out attack on Western Europe and the United States--without retaliation.

But the big difference, as far as the Soviets are concerned, is that the SS20s cannot reach the United States while the Pershing and cruise missiles can reach the Soviet Union. Thus, Moscow calls these new U.S. weapons "strategic" and claims that, far from being necessary to offset an existing Soviet advantage, they greatly tip the balance of nuclear power in favor of the United States.

The Soviets also argue that the United States, France and Britain also have hundreds of other missiles and bomb-carrying aircraft based in Western Europe that can reach the Soviet Union that should be counted in any negotiation over the power balance. The United States says that if such aircraft, for example, were counted, then all of Moscow's huge air force capable of bombing Western Europe should also be counted.