Cherry and Brad C are 100% correct, but I just want to add that this is really a "run-on sentence" and needs to be corrected.
Let me explain. There are two clauses: "form a union" and "an unorganized mob can accomplish only chaos." Each clause can be a complete sentence by itself: ("Form a union." and "An unorganized mob can accomplish only chaos.")
In this situation, you cannot join the two clauses with a comma. You can either:
1. use a semicolon
Form a union; an unorganized mob can accomplish only chaos.
2. add a word like "and," "but," "so," "because"
Form a union, because an unorganized mob can accomplish only chaos.
3. make it into two sentences
Form a union. An unorganized mob can accomplish only chaos.