Their natures are different and they operate in different spheres: NATO is a purely intergovernmental organisation functioning as a military alliance, which serves to implement article 5 of the North Atlantic Treaty on collective territorial defence.

The CSDP command and control structure is however much smaller than the NATO Command Structure (NCS), and the extent to which the CSDP should evolve to form a full defence arm for the EU that is able to implement the EU mutual defence clause in its own right is a point of contention.

The Western Union, established to implement the 1948 Treaty of Brussels signed by Belgium, France, the Netherlands, Luxembourg, and the United Kingdom, represents a precursor to both NATO and the EU's defence arm, the Common Security and Defence Policy (CSDP).

Since the end of World War II, sovereign European countries have entered into treaties and thereby co-operated and harmonised policies (or pooled sovereignty) in an increasing number of areas, in the European integration project or the construction of Europe (French: la construction européenne).

The following timeline outlines the legal inception of the European Union (EU)—the principal framework for this unification.

The EU inherited many of its present responsibilities from the European Communities (EC), which were founded in the 1950s in the spirit of the Schuman Declaration.

Had its founding treaty not failed to acquire ratification in the French Parliament in 1954, the European Defence Community would have entailed a pan-European military, divided into national components, and had a common budget, common arms, centralized military procurement, and institutions.

The EDC would have had an integral link to NATO, forming an autonomous European pillar in the Atlantic alliance.

Following the establishment of the ESDI and the St. Malo declaration, US Secretary of State Madeleine Albright were among others who voiced concern that an independent European security pillar could undermine NATO, as she put forth the three famous D's: Our [...] task is working together to develop [the ESDI] within [NATO], which the United States has strongly endorsed.

[9] The European External Action Service's (EEAS) Military Staff (EUMS), situated in the Kortenberg building in Brussels, has a permanent NATO liaison team and runs a permanent EU cell at NATO's Supreme Headquarters Allied Powers Europe (SHAPE) in Mons.

However it has been agreed that North Atlantic Treaty Organization (NATO) military structures may be used for the conduct of the EU's CSDP missions under the Berlin Plus agreement.

The EU command and control (C2) structure is directed by political bodies composed of member states' representatives, and generally requires unanimous decisions.