One Army Must Stand Guard
One Army Must Stand Guard is a setting that can be configured by game creators. When enabled, players must leave one army behind on every territory whenever they attack, transfer, or airlift armies out of a territory. Furthermore, when during an attack it is calculated that all attacking and all defending armies are defeated, instead one defending army will survive and stay in the territory.
For example, if you have 5 armies on a territory, only 4 would be eligible for an action, and the 5th must remain on the territory to guard it. However, if this setting is left unchecked, then all 5 armies would attack or transfer, leaving the territory from which you attacked with zero armies; however, you will still retain ownership(i.e. it has your color and counts towards your bonuses).
For example, you have a territory called Territory A and an enemy or neutral has a territory called Territory B. It currently has 10 armies. If you wanted to attack Territory B from Territory A and if:
One army must stand guard is enabled: 9 armies will attack Territory B and 1 army will stay on Territory A.
One army must stand guard is disabled: 10 armies will attack Territory B and you will have 0 armies remaining on Territory A.
As a last example, with the default kill rates and 0% Luck Modifier if only 1 army attacks it will kill 1 defending army, and if only 1 army defends it will kill 1 attacking army. However, with this setting checked, if 1 army attacks a territory with 1 army defending, the attacking army will be killed, but the defending army will survive, since one army must stand guard in the territory. Therefore it is not advisable to attack with 1 army with this setting on.
 Risks and Benefits of One Army Must Stand Guard Disabled
See this image for an example of one army must stand guard in action.
Enemies can easily take over all of you land, as territories do not have any resistance.
You can easily take over your enemy's territories, because they will more likely have fewer defenders and you will have more troops potentially available.