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.
I believe the flag should stay up because that memorializes the Civil War and the whole receding from the U.S.
