In 1960, a mysterious construction site in Israel’s Negev desert caught the attention of American officials. When questioned about the facility near Dimona, Israeli officials had a rather unusual explanation. It was a textile factory.But behind that explanation, Israel was building the foundations of a nuclear programme that would remain hidden from public scrutiny for decades. Washington knew about it, successive American administrations chose not to challenge it publicly, and Israel continued to neither confirm nor deny possessing nuclear weapons.More than six decades later, that carefully maintained silence is becoming harder to protect. The United States, which helped preserve Israel’s nuclear ambiguity for generations, is now facing questions over its position as it fights alongside Israel against Iran.With American lawmakers demanding answers, questions emerging over nuclear double standards and Saudi Arabia pursuing its own nuclear ambitions, the old understanding between Washington and Tel Aviv is facing a serious test.The latest CNN investigation by Davis Winkie, traces how this decades-old arrangement is now coming under pressure from multiple directions.In a June congressional hearing, US secretary of state Marco Rubio acknowledged that Israel’s nuclear capabilities are widely believed to exist, even though neither country has officially confirmed them.Responding to questions from Democratic Congressman Joaquin Castro, Rubio said, “I think most of the world assesses that they do, but we’ve never, they’ve never, acknowledged that publicly.” He added that Washington does not discuss the matter as part of its foreign policy.The questions are no longer confined to whether Israel possesses nuclear weapons. They increasingly concern the implications of maintaining silence while the US is directly involved in a war alongside Israel against Iran.Castro, who serves on the House foreign affairs and intelligence committees, told CNN that the policy of nuclear ambiguity had become “reckless and dangerous”.He questioned how Washington could fight alongside Israel without knowing its nuclear red lines or explaining what contingency plans were in place if those weapons were deployed or its nuclear facilities came under attack.The pressure has also moved to Capitol Hill. On May 4, more than two dozen Democratic lawmakers, led by Castro, wrote to Rubio seeking information about Israel’s nuclear capabilities, its potential use of nuclear weapons and the administration’s contingency planning.In their letter, the lawmakers warned that “the risks of miscalculation, escalation, and nuclear use in this environment are not theoretical”. They argued that Washington could not develop a coherent non-proliferation policy for the Middle East while maintaining official silence about the nuclear capabilities of a country fighting alongside it.Why Israel chose to keep its nuclear arsenal a secretFor Israel, maintaining nuclear ambiguity was a calculated strategy to enjoy the advantages of nuclear deterrence without facing the diplomatic and regional consequences of openly declaring itself a nuclear power.By neither confirming nor denying possession of nuclear weapons, Israel could keep its adversaries uncertain about its capabilities while avoiding international pressure and a potential arms race in the Middle East.A major concern was that publicly acknowledging its nuclear arsenal could encourage Arab countries to pursue similar capabilities and draw greater Soviet involvement in the region during the Cold War.A declassified July 1969 memorandum by then US national security adviser Henry Kissinger to President Richard Nixon reflected these concerns. Kissinger argued that Washington should prevent Israel’s nuclear possession from becoming an “established international fact”, fearing the consequences of openly acknowledging it.Israel also wanted to avoid international scrutiny, particularly as the Nuclear Non-Proliferation Treaty (NPT) took shape in the 1960s. The treaty, which opened for signature in 1968, recognised five countries as nuclear-weapon states under its framework. Israel never signed the treaty and continued to maintain ambiguity over its arsenal. Its approach helped it avoid the diplomatic complications that an official declaration could have triggered.Washington eventually became central to preserving this arrangement. While President John F Kennedy initially pressed Israel to allow inspections of its Dimona facility, the approach changed under President Richard Nixon.During a September 1969 meeting with Prime Minister Golda Meir, the two governments reached an understanding that Israel would not publicly declare, test or advertise its nuclear weapons, while Washington would avoid publicly challenging its nuclear status.More than six decades later, Israel continues to follow this policy, with the UK House of Commons Library estimating its arsenal at around 90 warheads.