The combined wealth of the non-US Allies, measured in GDP, is almost equal to that of the United States. This guideline also serves as an indicator of a country's political will to contribute to NATO's common defence efforts, since the defence capacity of each member has an impact on the overall perception of the Alliance's credibility as a politico-military organisation. In 2006, NATO Defence Ministers agreed to commit a minimum of 2% of their Gross Domestic Product (GDP) to defence spending to continue to ensure the Alliance's military readiness. These contributions are offered by individual Allies and are taken from their overall defence capability to form a combined Alliance capability, with each covering the costs associated with their deployments. Contributions can also include any kind of materiel, from armoured vehicles, naval vessels or helicopters to all forms of equipment or support, medical or other. For example, Allies can choose to contribute a few soldiers or thousands of troops to a NATO operation or mission. In all cases, NATO (as an organisation) does not have its own armed forces, so Allies commit troops and equipment on a voluntary basis. When the North Atlantic Council – NATO's top political decision-making body – unanimously decides to engage in an operation or mission, there is no obligation for each and every member to contribute unless it is an Article 5 collective defence operation, in which case expectations are different.
National (or indirect) contributions are the largest component of NATO funding and are borne by individual member countries.