Greek cities habitually resolved problems by either war or the arbitration by another city. Athens and Sparta had varied relations over several centuries, sometimes as allies, sometimes as adversaries.
Their main contest was the Peloponnesian War, when Athens was intervening against Sparta's allies, who demanded Sparta's support.
We today call this the Peloponnesian War 431-404 BCE.
In Scandinavia, against each other. And in Britain.
yes, many times
Occasionally different Chinook groups would fight wars against each other, or against other Northwest Coastal tribes.
Yes - anyone can fight anyone if they so choose. How could it be stopped?
Occasionally different Chinook groups would fight wars against each other, or against other Northwest Coastal tribes.
In World War II, they didn't fight against each other, they were on the same side.
Yes
Each other, Irish, Picts, Romans, Germans.
Afghanistan and Vietnam
Afghanistan and Vietnam
Civil War
Other knights for the most part. The occasional dragon or two, but usually other knights.