In book 8, the Achaeans found themselves on the back foot and the resurgent Trojans are feeling so much confidence that they camp outside of their mighty walls. This leads Nestor to pick out an embassy of Odysseus, Phoenix, and Ajax to try and persuade Achilles to return. They are only partially successful: Achilles will only return to fighting once Hector begins to gut the ships with fire. As The Achaeans cast about for another way to try to get the advantage over the Trojans and so they decide upon night raid. Diomedes and Odysseus go the Trojan camp and kill some newly arrived allies while stealing a number of majestic horses.
I also touch on the authenticity question around book 10 since a number of scholars argue that it is not by Homer.
An Introduction to Homer's Iliad Bk 10