There are several relevant Clausewitzian, Jominian, and Machiavellian concepts that would benefit Lord Gary. While some of these seem obvious to us, recall that most wisdom seems rather obvious in retrospect.
Clausewitz: War is an extension of politics, and wars have political goals. Putting 'war' in the 'politics' box doesn't reduce war, it expands the toolbox tremendously for achieving Lord Gary's goals. You can achieve goals through war...or through intrigue, marriage, etc. You can also use combinations of tools to achieve goals.
Jomini: Several Principles of War apply regardless of the technology: These principles vary a bit, but let's go with Objective, Offensive, Mass, Economy of Forces, Maneuver, Unity of Command, Security, Surprise, and Simplicity. This leads toward smaller, more mobile, better-synchronized combined arms operations.
Machiavelli: Motivate your troops directly, rally your population, use written guidelines of conduct, empower lower-level commanders to innovate. This will minimize the intrigue in Lord Gary's court, and will enhance the power of the force.
Plus one bonus with multiple authors:
- Military Staffs, a method of organizing information and synchronizing effort. It prevents overwhelming Lord Gary (or his Field Marshall) with unnecessary detail at the wrong times for smart decision-making. One goal is to reduce the decision cycle, so Lord Gary's forces that seem otherwise-equal-to-a-foe are nevertheless more nimble, survivable, and powerful due to the fastermore efficient information flow.
The tactical impact of this knowledge depends upon the resources Lord Gary is willing to bring AND upon their personality and preferences. If Lord Gary is willing to have influence (a measure of control) over neighbors, then ordinary, cheap intrigue might be enough. If Lord Gary is driven to exterminate the neighboring lords, erase the borders, crown himself King (or Emperor), then quite the bloody war may be likely...if he can afford the enormous cost.