DURING President Clinton's diplomatic debut in Europe, the military security of the West has already taken two potentially historic turns.
Both in cleaning up the dangerous nuclear residue of the cold war and in setting up a new concept of European security based on gradually integrating East and West, the United States has emerged smoothly into its traditional leading role.
The Brussels summit seemed to resolve the underlying question of whether Mr. Clinton was fully committed to a leading American role in Europe.
``I think everybody was impressed with the strong leadership, the resolve, and the conviction of the American president,'' said NATO Secretary-General Manfred Woerner at the end of the summit.
On the most immediate and concrete problem confronting NATO - the civil war in Bosnia - the leaders did little more than reaffirm their threat, five months old and still unrealized, to use airstrikes to end the Serbian siege of Sarajevo.
But Clinton also noted that private conversations with his NATO colleagues show that the political will is stronger than ever to carry out the threat.
The evidence of US leadership in Brussels came on two fronts.
When Clinton became president a year ago, he regarded the most serious security threat in the world to be the proliferation of nuclear weapons in the republics of the former Soviet Union.
On the president's first full day in Europe, US diplomats brokered an agreement by telephone to denuclearize Ukraine. The deal, to be signed with Ukraine Jan. 14, dismantles 1,840 nuclear warheads now targeted at the US and leaves Russia alone with the remainder of the old Soviet arsenal.
None of this was official business at what Secretary of State Warren Christopher called ``clearly one of the most important meetings perhaps in all the history of NATO.''
With remarkable ease, the NATO leaders agreed on inviting the former Soviet bloc and Europe's neutral countries to join the US-designed Partnership for Peace, a plan that opens the eventual possibility of NATO membership to the East. (US-European relations, Page 3.)
Western leaders see the Partnership as a template for forming the new shape of the Western alliance after the cold war. The overarching strategy, articulated here by Clinton, is to leave behind confrontation with the East and begin to integrate Central and Eastern Europe into a unified Europe based on Western institutions.
The most immediate action NATO will take to accommodate the East is to create ``combined joint task forces'' for taking on nontraditional missions that could include Partnership members. The first countries likely to join are those whose leaders Clinton will be meeting in Prague on Jan. 12: the Czech Republic, Hungary, Poland, and Slovakia.
Secretary Christopher said here in Brussels that he is confident that these four so-called Visegrad countries will move to join the Partnership for Peace in Prague by signing a Framework Declaration. Full membership in the Partnership will require legislative action in each country.
NATO leaders resist the concept that some Central European countries will be on a fast track to NATO membership. Any discrimination between applicants for the Partnership threatens to create new East-West divisions in Europe. But, as Mr. Woerner puts it, countries will pace themselves according to how quickly they can meet the requirements.
``Ultimately, the Partnership will lead to the enlargement of NATO and help us to build a security based not on Europe's divisions but on the potential of its integration,'' Clinton said at the end of the summit.
The deal to denuclearize Ukraine was ``one of the most important nonproliferation steps taken in years,'' Christopher said. It was worked out by US, Ukrainian, and Russian officials after Ukraine and Russia reached an impasse last summer and asked the US for help.
The catalyst seemed to be the prospect of a meeting between Clinton and Ukrainian President Leonid Kravchuk during Clinton's trip. The US made clear that Clinton would not meet Mr. Kravchuk without a deal. Clinton is scheduled to arrive in the Ukrainian capital of Kiev on Jan. 12.
US taxpayers supply some of the incentives in the deal, including $175 million to help pay for dismantling the weapons, which will then be shipped to Russia. The US will then buy the enriched uranium, worth about $1 billion, in a commercial transaction that involves no public funds.
The timetable has yet to be worked out. Ukraine is signing on to the seven-year maximum of the Nuclear Non-Proliferation Treaty. Belarus and Kazakhstan have already agreed to denuclearize and send their weapons to Russia, which will remain the only nuclear power in the old Soviet Union.
On Bosnia, NATO leaders added to their airstrike threat concerning Sarajevo a request for urgent military plans to open roads to Srebrenica and the air field in Tuzla.
Clinton strongly urged his colleagues on Jan. 10 not to renew their threat unless they were committed to carrying it out. The US was so committed, he said, and the renewed threat was included in the summit communique on Jan. 11.
European leaders have been concerned about the danger airstrikes would bring to their troops on the ground under United Nations command. But increasingly, Clinton said, the status quo is endangering troops as well.