The Acquisition and Cross-Servicing Agreement (ACSA) is negotiated on a bilateral basis between the United States and its NATO allies or coalition partners, allowing U.S. forces to exchange the most common types of assistance, including food, fuel, transportation, ammunition and equipment. The agreement does not commit a country to take military action. STAs also exist between third countries. Japan and South Korea have both formed ACSAs with countries other than the United States.  The Acquisition and Cross Service Agreement (ACSA) Act (formerly known as the NATO Mutual Support Act) was passed to facilitate the exchange of logistics, supplies and services between the United States and other NATO forces. It was amended in 1987 to allow CASA with the governments of eligible non-NATO countries, with further amendments in 1989 and 1990. It also requires equivalent exchanges (EEs) of logistical support, supplies and services and allows ACSAs with United Nations agencies and approval of equipment loans or leasings. Management reports are required, in which all ACSA transactions from the previous fiscal year are submitted and the requirements for the next fiscal year are presented. CASA authorities provide commanders and the service component or service orders with the means to acquire and provide mutual logistical support for training and travel, military exercises and operations, or to expedite access to the logistical resources of foreign forces to meet the logistical support requirements of deployed U.S. forces.
On 18 December 2014, the United States had CASA with 102 countries, 78 other CASA-eligible countries including most NATO countries, as well as NATO and the NATO Public Procurement Agency (NSPA), NATO Allied Command Transformation and Supreme Headquarters Allied Powers Europe (SHAPE). ACS reduces logistical effort and is considered important logisticians by providing site commanders with better interoperability, better availability and low-cost common support. CASA achieves this by creating a logistics supply mechanism between two parties in exchange for reimbursement either in cash or in lieu of equivalent goods or exchanges.