we can do nuclear fusion just fine. There are numerous research
experiments already doing it. Heck, there's even a small, but dedicated
amateur community setting up experiments. A while ago there was some
highschool kid who made the news by creating a small fusion device in
his living room.
The problem, however, is that maintaining a fusion reaction requires a
lot of energy, because the fusion plasma has to be kept at very high
temperature in order for the reaction to take place. In current
experiments, the amount of energy required to maintain the reaction is
considerably higher than the amount of energy produced by the reaction.
But, as it turns out, the amount of energy produced by the reaction
scales up more rapidly with size than the amount of energy required. So
by simply making the reactor bigger, we can increase the efficiency (the
so-called Q factor). But simply making the reactor bigger also makes
the reaction harder to control, so scaling up the process is not a quick
and easy job.
Scientists and engineers are currently working on the first reactor
to have a Q factor larger than 1. That is, a reactor that produces more
energy than it uses. This is the ITER project currently being
constructed in France.