English: Venn diagram displaying the historical sharing of nuclear weapons knowledge among declared nuclear weapon states (solid circles), undeclared nuclear weapon states (dashed circles), and South Africa, a former undeclared nuclear weapon state. The number of explosive nuclear tests performed is given in parentheses. Area of overlap is not strictly proportional to the degree of knowledge sharing, as this is difficult to quantify. The Russia-U.S. overlap reflects the recent purchase by the U.S. Defense Special Weapons Agency (via a detailed ne~otiated contract) of a large amount of data concerning the former Soviet test program. Transfer of information from nuclear weapon states to nonnuclear weapon states that would assist the latter "in any way' to acquire nuclear explosive devices is prohibited under Articles I and II of the Nuclear Non-Proliferation Treaty.