The obvious case is the consequence of climate change, where my emissions of greenhouse gases affects your sea level and climate (and vice-versa). aerial or marine pollution and overfishing in the oceans are others.
In a more restricted sense, one nation may hold all or most of a resource needed by all. Many nations are unable to feed themselves, at least to present standards, and therefore need to trade with others.
Convert to Christianity/Catholicism. When the Spanish started their conquering of lands, they always had a father on board so they could "save" the natives and convert them into their religion.
The people engaged in or available for work, either in a country or area or in a particular company or industry
Tiffany Stone
I learned about this last month in U.S History
