Let us look at the nuclear fusion example below to understand how the fusion reaction occurs.
When deuterium and tritium fuse together, their components are recombined to form a helium atom and a fast neutron. As the two heavy isotopes are recombined into a helium atom and a neutron, the leftover’s extra mass is transformed into kinetic energy.
The participating nuclei should be brought together for the nuclear fusion reaction to occur. They should be brought so close to each other that the nuclear forces become active and glue to the nuclei together.