This is like saying "The Czechs invaded the Sudetenland! Hitler invaded Czechoslovakia to get them OUT of the Sudetenland".
Kosovo wasn't "invaded" by Serbia: it was part of Serbia. Here's an analogy by way of explanation.
Imagine it's the year 2020. Mexicans are now 80% of the population of Texas and the Aztlanteans want to secede. President Jeb Bush sends in the National Guard to stop them. Did America just invade Texas?
depends on who wins. the victor writes the history