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.
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.
 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.