Optimal military size?

Getting the nunbers

Most countries now define the size of their armed forces from the size of their max projectable forces.
Say you want to be able to deploy a full brigade on a PeaceKepping (SFOR style) operation and sustain it. For that you need to have three Brigades at full strengh (One on mission, one trainning, one reserve)
For naval forces think deployments plus capabilities. you want to have one FFG avaiable for NATO operations at all times, you'll need to buy three of them. You want to have ASW, AAW, Patrolling, MW capabilities? You'll need Ships for that.
Air Force. You need enought fighters for the size of your air space, enought transport for the size of your airborne forces (and ressuply needs) etc.


The rule of thumb is:

You need three times the numbers you want to deploy on sustained missions
You need at least one third of the forces of a potencial invader (not to garantee a win, but to give you a fighting chance)
You need three times the forces of a country you are planning to invade.
 
AdA, what you're describing sounds like how Australia fought Vietnam. However the reserve unit isn't really formed, when it comes off its operational deployment it is more or less disbanded; long timers sent off on other postings, courses, remusterings, promotions etc, short timers discharge and then after some months the unit will be reformed from scratch and spend a year training for its next deployment.
 
AdA, what you're describing sounds like how Australia fought Vietnam. However the reserve unit isn't really formed, when it comes off its operational deployment it is more or less disbanded; long timers sent off on other postings, courses, remusterings, promotions etc, short timers discharge and then after some months the unit will be reformed from scratch and spend a year training for its next deployment.

Current misions are done in four month, or six month cycles in most countries. (the US retains one year deployments) with a 4/6 month cycle you need the 3 units to sustain operations. During the Vietnam era deployments were longer. The 3x rule apllies for professional armies for sustained effort (ISAF, KFOR, etc...)
The latest UK MOD white paper (on defence cuts) is a good exemple of how you do forces planning currently. (or the NL, or Belgium...)
3 is a minumum. A small army, with 3 brigades with 3 Battalions each can run two battalion sized ops at the same time (say one with ISAF and one with UNIFIL) but it will be stressed if it needs to deploy a third battalion on a sustained basis. I am, of course, talking of professional armies, or about the professional core that performs most deployments even in conscription armies.
The simplest example is SSBN. To insure that you have one on patrol at all times most nations assumed a need for 4. Of course you only need three crews, but most countries prefer to have a crew for each boat.
 
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The simplest example is SSBN. To insure that you have one on patrol at all times most nations assumed a need for 4. Of course you only need three crews, but most countries prefer to have a crew for each boat.

Or two, if you're the USN. Blue and Gold.
 
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