If you want to do well in war, it is important that you understand war weight.
Your war weight is a calculation of your base's total defensive and offensive capability, and it is the primary influence to the matchmaking system in Clan Wars. The matchmaking system will try to match one clan with another clan having similar strength, or war weight. Overall, clan weight is the combined weight of each base. Each base has it’s own war weight number that represents the base strength.
When people upgrade to Town Hall 9, they are often quick to build their Xbows. But why do that? Xbows weigh a lot, and if your walls are still level 8 (or even lower), a bad match could occur in Clan Wars. This is not Supercell’s fault; the war search is for two clans with the same combined war weight. Therefore, it is ideal to maintain a lower war weight to gain an advantage in the clan war matchmaking.
As a member of @ The Clan, your offence should always be prioritised before defensive upgrades. Please refer to the upgrade priority guide for a detailed list of the most effective buildings to upgrade for each town hall.