Final answer:
The Coulomb repulsion between atomic nuclei is the major obstruction to nuclear fusion. Nuclei must be given enough kinetic energy to overcome the repulsion and get close enough to induce fusion. One way to achieve this is by heating fusion fuel to high temperatures.
Step-by-step explanation:
The major obstruction to fusion is the Coulomb repulsion between nuclei. Since the attractive nuclear force that can fuse nuclei together is short ranged, the repulsion of like positive charges must be overcome to get nuclei close enough to induce fusion.
If the nuclei are given enough kinetic energy to overcome the electric potential energy due to repulsion, then they can combine, release energy, and fall into a deep well.
One way to accomplish this is to heat fusion fuel to high temperatures so that the kinetic energy of thermal motion is sufficient to get the nuclei together.