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