In the next four years, the focus of the war shifted to the south of the Balkan mountains. The Byzantines could not face the fast Bulgarian cavalry which attacked from different directions on a vast area. Towards 1194, Ivan Asen's strategy of swiftly striking in different locations paid off, and he soon took control of the important cities Sofia, Niš and the surrounding areas as well as the upper valley of the Struma River from where his armies advanced deep into Macedonia.