A shipping war is when two or more people have an argument over a paring of most likely fictional characters. Often fan art of the ship that lets say, person A likes will be sent to person B to upset them and
vis versa.
Shipping wars are either to prove the others ship wrong, to get the person to abandon their ship, to start shipping the pair you do, or simply to anger someone. Usually each person will bring up cannon situations and parts of the movie/book/show/manga/anime/etc to either back up why they ship that pairing or to prove why the other ship is irrelevant. Shipping wars can be long and
tedious especially if a large group is involved. If someone's OTP is involved the war will most likely be longer and will be more like a
flame war. If the same character is involved but each person ships them with a different character the war will be more hate filled and
canon will be used a lot. If your ship has liittle to no canon backing it up you will most likely loose.
Canon is an essential in a shipping war.
Example:
Let's say two people in the Attack on Titan fandom had a shipping war
Person A shipped
Levi x Eren but Person B shipped Levi x
Hanji. Because the character Levi is in both ships, conflict will arise. Person B would be at a canon advantage because there is more of a history and friendship with the characters. They could all so bring up canon examples of when the characters Levi Eren didn't get along. And if all fails bring in
the factor of age, Eren only 15 while Levi is 34. Person A won't have really any canon to back them up so the best they could do is resort to fanart to either annoy or get Person B to at least understand their ship.