Fresh water doesn’t come from Canada for most of the US. Additionally, it’s not nearly to the point of being a war-worthy resource. There would be plenty of water in California if farmers just paid for it the same way everyone else does.

California grows a lot of water-expensive crops (e.g. almonds) because legacy farmers don’t pay anything near what residents, commercial, or new farmers do. That would rebalance quickly and obviate the need for any invasion if we just dumped wasteful “water rights” and instead priced water like any other rivalrous good (e.g. govt auctions).