War declarations, like every other action, should be concealed until the turn resolves.
As it currently is there is a significant advantage to waiting to commit your turn until the last minute so you can declare war after your current ally has submitted. This is the same reason why infrastructure and warpgates purchases are no longer visible until the turn resolves.
It also means that when a player concedes a notification should not be sent to other players but instead wait until the turn resolves.
Alternatively, when a player declares war the effected players should have their turn submissions undone and the commit time reset to the current turn length. This allows players who login once a day to respond to war declarations.