The most fundamental unit of the Military Set is the "Lumber (Tier 1 Military Item)".
You can get any Tier 1 items (not just for Military Set) by playing the game (match 4+ items in any of settlements, or open treasure chests) or by buying them from the in-game store. Once you're further along in the Capital City, your higher tier buildings (such as "Wall") will also generate Tier 1 items (such as "Lumber").
When you combine three (or more) lumbers, you receive a "Fence (Tier 2 Military Item)":
Combining three (or more) fences will result in creating a "Wall (Tier 3 Military Item)":
Combining three (or more) walls will create a next tier building - a "Catapult", and so on.
Here is the progression list of buildings in the Military Set:
The rewards you receive from each set are based on the highest building you have completed (so if you have Lumber, Fence and a Wall, your rewards will be calculated based on the Wall because it's the highest level building), and they scale up accordingly.
Military Set will produce special items such as Fortune Cookies, Crates, Time Machines, and so on. The higher level buildings you have in the Military Set the better the rewards!