Leadership is not telling others what to do, its making them think they should listen. Its not whether others follow, but whether we make ourselves, by our actions, our judgement and our demeanor, worth following.
The best leaders do this without any need to resort to positional authority. That's not to say that positional authority can't make leading easier -- if nothing else it sets the expectation that we might be worth following -- but rather that position alone is not enough to make an effective leader - as anyone whose felt the need to assert their authority -- or been subjected to such treatment -- can attest.
Put another way, leadership is influencing others to act. Failure to do this means that one is not leading at all. The best leaders influence by instilling confidence in their choice of direction, the worst by fear of negative consequences if disobeyed.