In ancient times, there were not only scouts, but also spies, and Li Shimin was a master in this. When he and Dou Jiande were at Tiger Prison Pass, they sent people to Dou Jiande's army to inquire about intelligence.
The two confronted each other for more than a month, and one day a spy told Li Shimin that Dou Jiande wanted to attack Tiger Prison Pass when the Tang army ran out of horse food and grazed horses on the north bank of the Yellow River. This calculation was good, because Li Shimin's cavalry Xuanjia Army was one of the strongest soldiers at that time, and if there were no horses, this army would not have such strong combat effectiveness.
Li Shimin wanted to lead Dou Jiande to battle, and on May 1, Li Shimin crossed the river to observe Dou Jiande's battle front, and when he returned, he deliberately left more than a thousand horses to graze by the river, and sneaked back to the Tiger Prison Pass at night.
In ancient times, there were scouts in wars, called scouts, Dou Jiande's scouts detected the Tang army herding horses by the river, speculating that their horse food had been eaten, so the next day Dou Jiande led a large army to attack Tiger Prison Pass, which was indeed too heavy to hold its breath, and the Tiger Prison Pass station was the most wonderful battle in the history of Li Shimin's military use to win more with less.
Reference: Old Book of Tang