The Cadorna barracks in Legnano, outside Milan, is buzzing with activity as 529 Italian infantrymen make final preparations for their expected departure for Lebanon as part of a projected international peace-keeping force.

Like Italy's enthusiastic participation in the larger Sinai observer force, its involvement in Lebanon is seen here as an indication that after years of inertia, Italian foreign and defense policy has taken on a new, activist sheen.

Italian participation in the three-nation force, which also would include American and French troops, was approved here last Friday in one of the last political acts of caretaker Premier Giovanni Spadolini, whose five-party coalition resigned the next day.

But the formal go-ahead was just the last act in several weeks of intense diplomatic activity by Italian Foreign Minister Emilio Colombo. A Foreign Ministry official said this week that Italy first began considering the possibility of participating after the French offered their own military contingent earlier this summer. Both the French and the Italians already had a small military presence in Lebanon as part of the United Nations peace-keeping force there.

In July, Colombo, a Christian Democrat, discussed the matter when he met in Washington with Secretary of State George P. Shultz and in Rome with U.S. special envoy Philip C. Habib. According to diplomatic sources, Colombo told the Americans that Italy was willing to supply a military contingent as long as other Europeans participated and the Israelis, the Palestine Liberation Organization and the Lebanese all agreed.

The Italian troops are ready to fly to the southeastern Adriatic port of Brindisi to sail for Lebanon on two Navy ships. Western diplomatic sources said Italian Defense Ministry officials are in Beirut helping to work out final arrangements.

The participation in a multinational peace-keeping force of the highly trained Italian troops, known for their black, feathered helmets and their unique running march accompanied by trumpets, is a source of pride for Italian policy-makers.

It also is seen as a sign of Italy's new assertiveness.

There also has been a new willingness to assume greater responsibility in the Mediterranean area as well as unprecedented increases in defense spending and some uncharacteristic, innovative strategic changes.

Two years ago, the Italians responded to an appeal from Maltese leader Dom Mintoff--worried about growing Libyan pressure -- with a guarantee of the island's neutrality and promises of substantial economic aid.

Arms sales to Libya have been curtailed sharply. And Western diplomats and military officials have words of praise for discreet Italian attempts to ease Greek-Turkish tensions on NATO's southern flank.

Furthermore, although the Italian defense budget, at 2.5 percent of the gross national product, is still the lowest in NATO, outlays have risen sharply over the last three years.

Renewed international and East-West tensions are credited with at least some of the responsibility for Italy's new decisiveness. A breakdown in domestic detente between the dominant Christian Democrats and the Communists also has played a role.

But much of the burst of dynamism, including an energetic policy of armed forces modernization, has been the work of Defense Minister Lelio Lagorio, the first Socialist to hold that post.

Because of the Socialist Party's past tradition of neutralism, Lagorio's appointment in April 1980 at first caused anxiety in Washington and in NATO outposts in Europe. But Lagorio, 56, a former mayor of Florence, has worked closely with party leader Bettino Craxi to bring the party on to firmly pro-NATO ground.