>>96029645
Yes. The Deathwatch are the chamber militant of the Ordos Xenos. Theoretically any Inquisitor with enough influence and cause could requisition a watch fortress to rally to a particularly important major battle. While they are normally only requisitioned in ones or twos, if an entire planet was falling to Tyranids in range of their watch fortress they would absolutely go to war as an entire army.

In terms of being allied to any army, its because they are being dispatched as assets by the Inquisition to aid an Imperium army or showing up themselves either on their own or after being asked because of their expertise in fighting Xenos.

In the lore the Deathwatch have deployed as full companies plenty of times. I recommend the Deathwatch Omnibus.