Their kingdom was taken away, their money was usurped, they were sent into the forest, so many tribulations. But the foolish Duryodhana did not know that above all, there was Kṛṣṇa on their side. That he could not calculate. Therefore when he saw the Pāṇḍavas are well-equipped with good number of soldiers, pāṇḍavānīkam, he was little surprised, that "How they could gather are so many soldiers?" So immediately, to consult the commander-in-chief Dronācārya....
Dronācārya was everyone's teacher, ācārya. Ācārya means teacher. Ācāryam upasaṅgamya (BG 1.2). The military teacher. So Duryodhana, he was military teacher of the Pāṇḍavas also. When all of them were children, they were given under the instruction of Dronācārya.