Unrelenting, Milosevic undertook the massacres of the last year, which finally precipitated NATO's bombing.
Without relent, Milosevic undertook massacres of last year which on their own led to NATO bombing. 
mnli hypothesis: Without relent, Milosevic undertook massacres of last year which on their own led to NATO bombing.  premise: Unrelenting, Milosevic undertook the massacres of the last year, which finally precipitated NATO's bombing.