cross someone's T

From the shape formed by the opposing battle lines, which resembles a capital letter T.

cross someone's T (third-person singular simple present crosses someone's T, present participle crossing someone's T, simple past and past participle crossed someone's T)

1. (military, nautical) To sail one's battle line across the front of the enemy battle line, allowing one to bring one's full firepower to bear on the enemy battle line while limiting the firepower that the enemy can use in return.