Even after the removal of Soviet and American medium-range nuclear missiles, the Eastern and Western blocs still confront each other in Europe with 4.6 million troops, 74,400 tanks, thousands of aircraft and a bristling array of nuclear weapons.

"The concentration of forces in Europe is the largest, the costliest and potentially the deadliest peacetime military confrontation in human history," says the Union of Concerned Scientists, which lobbies for cuts in military spending.On Thursday, 23 nations of East and West will meet in Vienna for the beginning of what may be several years of negotiations on reducing conventional - that is, non-nuclear - military forces in Europe.

Secretary of State James Baker flies Sunday to Vienna, where Soviet Foreign Minister Eduard Shevardnadze is to attend the parallel Conference on Confidence- and Security-Building Measures and Disarmament in Europe. The two chief foreign policy officials will meet Tuesday for the first time since Baker took office.

The meeting is expected to be followed by a more substantive get-together in Moscow during the spring that may pave the way for a summit between President Bush and Soviet President Mikhail Gorbachev.

But State Department officials said Baker and Shevardnadze would have detailed discussions in Vienna about the Soviet minister's recent visit to the Middle East and other issues.

The negotiations between the North Atlantic Treaty Organization and Warsaw Pact nations that open Thursday are being called the Conventional Armed Forces in Europe, or CFE, talks.

For the past 15 years, a more limited group of nations from East and West negotiated the same issues in a conference called the Mutual and Balanced Forces talks.

Those talks bogged down over Western claims that the Soviet bloc has vastly superior conventional forces in Europe, and the Warsaw Pact's insistence that the two blocs are roughly equal in strength. Western arms experts said the talks failed because the Soviet bloc never had any incentive to negotiate away its advantage over the West.

But the new talks are opening with hopes much higher. Western experts said Gorbachev must scale down his military establishment to achieve the financial savings he needs to prop up his sagging economy.

A new willingness by the Soviets to allow verification of arms agreements, something they stubbornly resisted for years, also has contributed to greater optimism.

And Western Europeans, perceiving a diminished threat from the East with the 1987 treaty to remove medium-range nuclear missiles, are bringing heavy pressure on NATO for an agreement.

In his United Nations speech in New York last December, Gorbachev fired a pre-emptive shot by announcing plans to reduce Soviet forces by 500,000 over the next two years, eliminate six tank divisions with about 2,000 tanks and withdraw 3,000 more tanks from Eastern Europe.

Gorbachev said Soviet forces would be transformed from an offensive to a defensive alignment.

East Germany, Poland and Hungary then announced plans to reduce their forces and their military budgets.

These moves give a propaganda advantage to the Soviet bloc going into the talks, and NATO appears not to have helped its position by its opening proposals on troop cuts, announced last December in Brussels.

NATO has called for reducing tanks, artillery and armored troop carriers on both sides to an equal level that would be about 5 percent below NATO's current holdings. This would mean a reduction of about 2,000 NATO tanks to 27,000 for the Warsaw Pact, even after the unilateral Soviet cuts Gorbachev announced.

The proposal was a compromise among NATO allies, led by West Germany, that want to make deep cuts, and others such as France and Britain that are less enthusiastic. The proposal does not mention any cuts in combat aircraft or surface-to-surface missiles, and it has been widely criticized as too limited in scope and certain to be rejected by the Warsaw Pact.