Swiss Cheese Units is a term made popular in 2006 to describe the state of military National Guard units in the US. In general, a guard unit trains together and when it is called for, will be deployed together because they have developed the ability to work together as a team. Also, the military unit has a certain amount of equipment allotted to it.
However, in order to strengthen forces in Iraq, individual members of a guard unit may be deployed. This has led to the term Swiss Cheese Units to express that the remaining guard unit is full of holes because it is not completely staffed.
Not only people, but also equipment, may also be taken from individual guard units in order to replace equipment lost in Iraq, or to boost equipment needs. So not only may Swiss Cheese Units be missing people and weaker, but the Swiss Cheese Units may also lack important equipment that is vital for training and practice. This can create guard units that are ill prepared to be deployed, and would not fully be able to assist in military efforts within the US.
Some of the problems associated with deploying certain members of a National Guard unit existed before invasion of Iraq and Afghanistan. National Guard Swiss Cheese Units were not at full strength even before these military actions, making them vulnerable when losing even one member.
Further, new military policies enacted by President Bush called for longer durations of service for all military units. Service prior to 2006, in any military engagement, was essentially set aside and all National Guard units were told they would now be expected to serve one year in Iraq, or in other active military engagements, per every five years in the Guard. This allowed the government to immediately deploy more troops, leading to the Swiss Cheese Units effect.
What concerns many who work in National Security, and critics of the Bush administration is that Swiss Cheese Units create more risk within the US. By not having fully staffed guard units with appropriate equipment, a terror threat or attack in the US might not be met with the full strength of the National Guard. Swiss Cheese Units would have to make up for their deployed members, which could weaken army response in an emergency situation.