Unless you define what you mean by “kiruv”, you will get many different answers depending on how the person answering defines “kiruv”. Several hundred years ago, it was common for Jews to be restricted legally as to where they could live, what professions they could engage in and who they could marry. There was no social welfare from the state. If you needed medical or financial assistance, you were most likely to find it being given by other Jews in your community. The ability to freely choose a lifestyle was not common until after the “enlightenment” in Europe.
After WWII, the work week changed from six days to five which made it easier to keep Shabbos.
The creation of the state of Israel and the six day war were electrifying events. These were viewed by many who witnessed them (even Gentiles) as the fulfillment of the Biblical prophecy that Hashem will return the Jews to the land of Israel at the end of time. If the Bible made a prediction about the future that came true despite the fact that it was against all odds, that was clearly a wake-up call from Hashem that the Torah is true. So my answer is that Hashem started the kiruv movement. The Bible says that Hashem will circumcise our hearts. Why not simply say Hakol Mishamayim?