Framework of the Book "We must be the great arsenal of democracy", 1 Franklin D Roosevelt announced on December 29, 1940, at a time when National Socialist Germany had occupied much of Europe and the United Kingdom was increasingly under pressure from the Germans. Winston Churchill proclaimed: "Give us the tools, and we will finish the job. " 2 Soon thereafter, what had been a figurative slogan became reality. The US launched the Lend-Lease program. It still kept clear of the actual fighting. But it was supplying substantial military aid to allied States fighting National Socialist Germany. It literally became the 'arsenal' of States defending democracy against National Socialist Germany. Josef Stalin later noted at a dinner in Tehran "[w]ithout American production, the United Nations could never have won the war. " 3 46 years later, in 1986, the United States conducted airstrikes against Libya in an operation that has been described as the "longest and most demanding combat mission" in US military history. 4 The reason: European and regional States had denied their support, neither allowing American aircraft overflight nor refueling. In 2019, a German court determined that American drone strikes in Yemen are only made possible due to the use of a relay station based in Ramstein, Germany. 5 Recently, the involvement of third States in the Ukraine conflict defines the ongoing war.Interstate assistance to use of force matters. These four examples are no exception. In fact, it is rare for States to use force in their international relations without assistance from another State. In view of a use of force, States cooperate. States provide each other with security assistance, long before they resort to force, by training soldiers, exporting arms, or joining