According to Rav Shlomo Goren, z'l, former chief rabbi of the Israeli Defense Forces and then of the State of Israel, there are times when G d must introduce His will into the hearts of people, but this is limited to monarchs, emperors, and Pharaohs.
Individuals always have free choice.
However, since G d has a master plan with Israel as the catalyst, the Almighty may sometimes be moved to control the choices of leaders of key nations during critical and fateful historical periods. Such a situation occurred at the very dawn of history with the confrontation between Pharaoh and the Hebrew slaves, and the Almighty had to step in. A second way of looking at the issue is provided by the Midrash. True, G-d hardens Pharaoh's heart as He declared He would, but the Divine intervention only emerges with the sixth plague. Examining the first five plagues we find that Pharoah himself is the one who exercises obstinacy.