In the month before the ancient Olympics no wars were permitted so that spectators could travel from across Greece unharmed.
When Vietnam invaded Cambodia, the Cambodian king sent a beautiful woman who convinced the Vietnamese commander to withdraw. The Vietnamese then sent a famous general, who also withdrew after meeting the same woman.
Last time when US president formally declared a war supported by Congress was in 1942. Everything since were conflicts without war proclamation.
In 1859, an American farmer in the San Juan Islands shot a pig who was eating his potatoes, but the pig happened to belong to a British colonist. In the ensuing confrontation, 461 Americans and 14 artillery faced 5 British warships with 70 cannons and 2140 men. The only casualty was the pig.
In the 1300s, some fellows from Modena stole a bucket from Bologna (both in Italy), resulting in a great deal of humiliation for the Bolognese. They declared war, had a battle with around 2,000 casualties (split between both sides), and failed to reclaim the bucket.
Andorra declared war on Germany during WW1, but didn’t send any soldiers because they didn’t have an army. At the Treaty of Versailles, Andorra was forgotten and technically remained at war with Germany, until the two countries declared peace in 1958.
The city of Huescar forgot they were at war with Denmark until a local historian found the declaration 172 years later. No shots were fired during the war.
In 1974 Jane Goodall observed a social rift in a community of chimpanzees turn into a violent 4 year civil war for territory involving kidnapping, rape and murder, changing her perception of chimpanzees, it is known as the “Gombe Chimpanzee War”.