In the original Transformers trilogy, the Autobots were eventually accepted as allies of the human race (and the U.S. Military in particular) and Duhamel’s Captain William Lennox fought alongside them; both during his service with the U.S. Army in the first two films and as a member of the joint human/Autobot NEST task force in Transformers: Dark of The Moon. However, post-Age of Extinction, the Autobots and Decepticons alike are fugitives from a paranoid humanity that hates and fears them. Thus, Lennox has a for-now unclear relationship with the Transformers Reaction Force (TRF): a paramilitary unit assigned to hunt and destroy all alien robots.
Duhamel had the following to offer, with regard to Lennox’ absence during the events of Age of Extinction – as well as the character’s relationship with Santiago Cabrera’s ruthless TRF operator, Santos, in The Last Knight:
Well, the last one, to sort of justify it to myself, there was no military element. Therefore they didn’t need me. I think it was probably they just didn’t want me (laughs). But this time there is definitely a military element here and what brings me back is, there’s a question as to who I’m working for. Am I with the US military, the Army, or am I working with a group that’s trying to eradicate all Transformers? Not just Decepticons, but all of them. There’s a bit of a struggle between myself and Santiago Cabrera, and then we’re also working against [Mark] Wahlberg’s character in this. Because he’s definitely working with the Autobots. It’s just a lot of big fun movie s–t that we get to do. And what brought me back personally is the fact that you get to do stuff that you don’t get to do anywhere else. Michael [Bay] does stuff bigger than anybody I’ve ever worked with for sure. I’m a tiny cog in this machine. I get to see some really cool s–t.
As for the NEST, Duhamel confirmed that the group is not a part of the story in The Last Knight:
NEST is not in this. There’s a different group. NEST was a group where we worked with the Autobots. It’s so crazy to talk about. It’s all based on a true story by the way. But this group, there’s sort of a worldwide coalition to eradicate all of them. They’re all bad. And a few of us know better, but that’s sort of it.