The most practical example is colonial era. The most of Africa was conquered without much of a fight. You can see British in India where most of the states entered subsidiary alliance for help against each other.
The main source by which this is achieved is prestige. How much prestige a nation has easier it is to win. Take German unification, it is the prestige of Prussia which made other nation join her to form German empire. Prestige necessarily doesn’t mean strength. Sometimes past glory developed society will contribute to prestige and victory over stronger nations. Ex-colonial European powers are example.
Another is economic influence, if you have enough economic influence you can win over many nations by exerting economic pressure. Economic sanctions by US and EU. is prime example.