During times of peace, Vineta had a total of just six thousand troops in its Standing Army, and to save money, they were dispersed to be stationed in various cities since supplies could be provided by the cities where they were based.
Therefore, Antonio and Layton could not afford to waste Vineta's elite soldiers at the gates of Tachi, but they also could not afford to leave an Alliance army poised to strike in their rear.
That was the reason Layton cursed the Tanyrians for stealing their own secrets. It was an era where "tactically, offense overwhelmingly beats defense, but strategically, defense overwhelmingly beats offense." When your own trump card for victory was used against you by others, it would frustrate anyone.
However, there was nothing to be done about it because humans learn everything else slowly but pick up the art of warfare exceptionally quickly.