PARIS, April 15 (AFP) - NATO formally celebrates its 50th birthday later this month embroiled in its biggest military operation since it was created.

The past 23 days have seen a constant escalation of its involvement in the Balkans.

Here is a chronology of events:

March 24: NATO launches air strikes on Yugoslavia with the goal of crippling the Serbian war machine in Kosovo and enforcing compliance with the international peace plan drawn up at Rambouillet, France.

March 25: US Stealth bombers join the air campaign and Belgrade orders the media from NATO countries out of Yugoslavia.

March 26: Tomahawk cruise missiles are fired in the first daylight attacks. The first of a massive tide of refugees arrive in Albania.

March 27: A US F-117 Nighthawk Stealth fighter is shot down near Belgrade but the pilot is recovered.

March 28: NATO launches phase two of its campaign, directly targeting Yugoslav armed forces. Western leaders accuse Yugoslav President Slobodan Milosevic of "genocide".

March 29: A NATO official says that operations against Yugoslav military targets are in progress "round-the-clock" in a race against time to prevent Milosevic "cleansing" Kosovo of its ethnic Albanian population.

March 30: A mission to Belgrade by Russian Prime Minister Yevgeny Primakov produces an offer by Milosevic to negotiate that western leaders say fall well short of what is required. Serb forces ship Kosovars to the Macedonian border in sealed trains.

March 31: Three US soldiers are snatched by Serb forces after an incident on the Macedonian border. The exodus of Kosovar refugees speeds up.

April 1: The US soldiers are paraded on Serb television. Moderate Kosovar leader Ibrahim Rugova is shown on Serb television apparently calling for a "political solution." NATO warplanes destroy a bridge over the Danube at Novi Sad.

April 2: NATO dismisses calls for an Easter truce as US B-1 bombers are used for the first time. The refugee crisis in northern Albania and Macedonia escalates.

April 3: NATO air strikes hit the Serbian and Yugoslav interior ministries in central Belgrade.

April 4: Western military transport planes launch an air-bridge operation, ferrying food, medicine and tents to Albania and Macedonia.

April 5: Improved weather conditions enable NATO to launch the heaviest air strikes so far. The Pentagon says its detects signs of "impact" on the Serb army.

April 6: Belgrade announces a unilateral ceasefire, immediately rejected by NATO as "insufficient." NATO warplanes stage their most ferocious attack on Yugoslavia to date.

April 7: Yugoslavia seals its borders with Albania and Macedonia. UN Secretary General Kofi Annan warns that the perpetrators of suspected "genocide" against ethnic Albanians in Kosovo will not go unpunished.

April 8: Cypriot parliamentary speaker Spyros Kyprianou arrives in Belgrade in an attempt to get the three caputred US soldiers released. NATO says it may target Serbian radio and television facilities.

April 9: Russian President Boris Yeltsin warns NATO powers they risk a European or possibly world war if they attempt to "seize Yugoslavia." Fighting involving Serb troops is reported on the Yugoslav-Albanian border.

April 10: Russian Foreign Minister Igor Ivanov says Moscow has no plans to enter the Kosovo conflict. Kyprianou returns to Cyprus after failing to secure the release of the three US soldiers.

April 11: US lawmakers call on the White House to start planning for the possible use of ground troops in Kosovo.

April 12: British Prime Minister Tony Blair says the Rambouillet accords providing for Kosovo's autonomy within Serbia are no longer a viable option. The parliament in Belgrade votes to join a Russian-Belarus Union.

April 13: US Secretary of State Madeleine Albright meets Ivanov in Oslo but they remain at odds over the proposed deployment of international troops in Kosovo. Yugoslav forces mount a cross-border attack on a village in north Albania.

April 14: Tirana says it is sending reinforcements to the north of the country to guard against further incursions by Yugoslav forces. Yugoslavia claims that rockets fired by allied jets killed 75 people in two separate refugee columns in western Kosovo.

April 15: NATO pursues its blitz on Yugoslav communications and industrial infrastructure. Serb shells fall around the main border post between Kosovo and Albania, at lesat one falling inside Albania.