It was going to be a bloodbath, the war was going to induce horrendous casualties.
Rex knew that from the start, the impending battle against the Executor would be his hardest yet, and he wasn't going to let his own forces and the allied nations participate in this fight—potentially even crippling them for a long time.
Disregarding his will to take down the Executor, this was not the only problem at hand.
Even if he did defeat the Executor, the war will continue.
Killing the Executor would only be opening the door of hope that peace might be realized.
Because of that very reason, his forces and allied nations have to remain strong.
He was open about everything to the other commanders.