Use the verb unite to describe two or more things merging into one, like several angry citizens who unite to form a political group.
If two countries come together to form one country, they unite. This happened in 1990 when East Germany and West Germany merged into one united Germany. If several people act as one, joining for a common goal, they also unite. The word has been used in English since the fifteenth century, and it came from the Latin unitus, "to unite," which in turn has its roots in unus, which means "one."