Bhagavad-Gita 1-3

Paschaitaam Panduputrena aachaarya mahateeM chamoom

yooDaam drupada putrENa tava shishyEna dheemataa ||3||

"Look at the mighty army of the Pandavas which has been organized by your able pupil the son of Drupada"

Drishtadyumna - the son of Drupada was the Commander-in-chief (CIC) of the mighty Pandavas army. Drupada and Drona were childhood friends when they were at the gurukul of Sage Bharadwaj. Drupada had promised him half his kingdom once he occupies the throne. Later when Drona turns up and asks for half the kingdom Drupada humiliates him and throws him away.  Drona temporarily retreats but did not forget the humiliation. Drona trains Pandavas and with the help of them defeats Drupada, holds him as prisoner, takes away half the kingdom and humiliates him before he releases him. Drupada performs Putrakaameshti Yagna and gets Drishtadyumna as his son who was destined to kill Dronacharya. Though Drishtadyumna was destined to kill Drona, he did not turn him away when he came to his gurukul to learn from him as he thought it was his duty to teach anybody who came to his doors to learn. Duryodhana tries to freshen up the old wounds when he reiterates that the CIC of Pandavas army is Drupada's son.  He probably wanted to say that Drona was not just fighting for Duryodhana but was fighting his personal war too. He also reminds that he should not be generous again in this war against his enemies who were his pupils.

{* The words Aneekam and Chamoom have been used for describing the army of Pandavas. The composition of the armies is described as in the table below:

Name for Army Stages	Charriots	Elephants	Soldiers	Horses
Patti	1	1	5	3
Senamukha	3	3	15	9
Gulma	9	9	45	27
Gana	27	27	135	81
Vahini	81	81	405	261
Prutana	261	261	1215	783
Chamoo	783	783	3645	2349
Aneekini	2349	2349	10935	70470
Akshouhini	23490	23490	109350	704700