"Why did NATO suddenly decide to do this? Why defend Kosovo Albanians?"
These were the questions asked by a friend in 1999, when NATO bombed Yugoslavia to forc it to sign the agreement that allowed the organization to turn Kosovo, a province of Serbia (then part of Yugoslavia) in a US protectorate and EU.
My answer, written in 1999 after the end of the bombing was:
I've been worried about the Kosovo crisis like you, and I want to use you as a judge of a thesis I am developing. My position is based on the information extracted from the US State Department, Stratfor, the Library of Congress, the UN, NATO, the press, and other sources.
Justification for War
To justify the war, Clinton and Blair sold to the public the idea that the NATO intervention in Kosovo was intended to rescue the Albanian population of that region. They had to save this Muslim population (almost one million people) from a genocidal campaign conducted by Christian Serbs. They also said they wanted to offer a measure of autonomy to Kosovo, and pledged that the rights of the Christian minority in Kosovo would be protected.
The problem, as I see it, is that genocide did not occur, was just an excuse, and NATO actions precipitated the result that Clinton and Blair said they wanted to avoid.
The initial strategy fails
Clinton also made a mistake when he assured the US public neither army troops nor marines would be used. Clinton thought that the bombs would do the trick. But Milosevic, knowing that troops were not on the game board, thought the NATO bombing would end when they realized that the Serbs were unwilling to surrender. Both sides miscalculated.
The bombardment began, the Serbs did not give up. They never even remotely thought of allowing control of Kosovo by NATO. Instead, they took revenge on the Albanians. They began the forced expulsion of the population in late March and early April after the bombing began. Albanians were injured, killed and abused. Hundreds of thousands of people were forced to flee their homes. Milosevic, with the full support of the Serbian people, was telling NATO go to hell.
Why the Serbs did not give up
NATO did not understand that the Serbs would resist, because Kosovo was, and still is, an integral part of their nation. Kosovo was carved from Serbia using brute force. Today the Serbs are a minority in Kosovo because the Albanians immigrants have taken over the region. However, the Serbs still believe that Kosovo is theirs, and that they are the victims of Albanian aggression. They think they made a mistake when they allowed their numbers decrease in Kosovo, while the Albanian Muslims increased their numbers to become the majority.
The NATO war crime
Failing the initial plan, General Wesley Clark came up with the idea of bombing "military" targets located within areas full of civilians. Clinton and Blair, knowing that the new plan was to kill large numbers of innocent people, approved the idea to bomb government buildings full of civilians and / or located in densely populated areas.