In the middle of the sun, where it is 10 of millions of degrees and trillions of atm pressure, hydrogen atoms are forced into combining and making helium atoms, during which huge amounts of energy is released. This is the principle of the hydrogen bomb. So we have done this on earth, but in an uncontrolled manner.
We can't get trillions of atmospheres pressure, so we compensate by using lower pressures but higher temperatures, and using different types of hydrogen, deuterium and tritium. Still hydrogen, but they have neutrons while hydrogen doesn't. The trick is containing the atoms at these temperatures and pressures while the reaction takes place, and this has been achieved in tiny scales using lasers (I don't know, can't even begin to imagine, how) and the things go bang and enough energy to power a wristwatch is the result ....
So the holy grail of fusion is being able to scale it up so you can get commercial quantities of energy cheaply. Its big advantage (if it can work) over current nuclear power stations is that it has negligible nuclear waste, and the deut and trit are extracted from sea water. All the research has been done using big magnets and electricity and lasers and things, but to no avail yet. If Lockheed can make a 100MW generator the size of a Holden ute - then problem solved.
