I believe that the war against Belgrade in 1999 should not have been supported by the left for two reasons.
First, the war provided the time, space and setting for the intensified slaughter of the Albanians of Kosovo.
Second, it was only on account of good fortune that after two months of aerial bombardment Milosovic capitulated and agreed to withdraw from Kosovo on condition that his Belgrade regime was left alone. In the absence of a ground invasion it was always possible for the regime to have remained defiant.
Had Milosovic been threatened at the outset with a clear ultimatum: hand Kosovo over to the EU (not NATO) or face a ground invasion, then the war would have been worthy of support: except of course it would never have happened.