People have been hunting whales for at least a thousand years, and there were no real problems until the 20th century. What happened was that fishing technology became much more efficient and the ships were much faster, so more and more whales were caught. In the 1960s the main whaling countries were killing more than sixty thousand whales a year, and everyone began to realize that something had to be done.
It was environmental groups like Greenpeace that really made things change. They set up out to make people aware of the fact the whales were fast becoming extinct. But even now we don't know if all this interest has come too late. Let's take the great blue whale for example, which at thirty to forty metres long is the biggest animal there has ever been - now there are perhaps about two thousand or so left. In fact they have been protected for quite a long time, but there is still no sign that the population is growing.