Another measure for combat effectiveness is developed by Youngwoo Lee and Taesik Lee who use a "meta-network representation" approach with regards to the opportunities available for military units to make an attack. The number of enemy casualties is one of the main indicators of success in combat and was used by the United States army in the Vietnam and Korean wars. According to Lee and Lee, there are two types of direct engagements with enemy forces in the network model: isolated attacks and coordinated attacks. Let formula_10 and formula_11 be two friendly force units and formula_12 be an enemy unit. In an isolated attack between formula_10 and formula_12, formula_10 must carry out the responsibility of both detecting and advancing on formula_12. On the other hand, a coordinated attack allows formula_11 to communicate the detection of formula_12 to formula_10 if formula_10 does not have the capability or is not in a position to detect formula_12. If formula_10 is in a position to engage formula_12, formula_10 may carry out the attack through the organisation between the two friendly forces. Lee and Lee say that more complex combat situations can see these networks expand to include more combat units, locations, capabilities and actions, but the base structure is of an isolated or coordinated attack structure. The larger the network, the greater chance of opportunities for offensive action to become available.