The FBI made Whitey as powerful as he was. He operated with immunity for decades which added to his "legend" on the streets. As his legend grew, so did his power but it wasn't true power IMO, it was perceived power. He prayed on the weak independent guys who had no where to run. In reality, he was nothing compared to the Italians in the North End but their perception of him was much greater than reality and his power/legend snowballed from there.