There is a 24 hours cool down period when a player joins a guild. If any guild contest, events, or wars start during this 24 hour period the player will not be eligible to receive rewards. Only events that start after the 24 hour period will be able to provide rewards. The player must also have participated in a War in order to be eligible for rewards. This is done to prevent guild hopping.
A player can only earn war boxes for each promotion once per season. If a player has already earned the bronze war boxes, and then joins a guild that is promoted to bronze, the player will not get additional war boxes.
Legendary: You will get additional war boxes at the end of the season based on your ending rank. The better the rank the better the rewards!
If a player leaves a guild or is kicked from a guild during war, they keep any war boxes they've already earned from previous promotions. At the end of the season, the player will receive the season rewards for the guild they're currently in, provided they participated in at least one war with the new guild during this season.