The last major nuclear weapons control treaty between the United States and Russia is set to expire on Thursday, raising concerns over a possible new arms race.

The Strategic Arms Reduction Treaty, known as New START, signed in 2010, capped deployed strategic nuclear warheads for each country at 1,550. The treaty also included transparency measures such as data sharing, notifications, and on-site inspections to reduce the risk of miscalculations.

Experts warn that its expiry could mark the end of decades-long arms control cooperation that helped maintain global security since the Cold War.

Pope Leo has urged both nations to renew the treaty, noting that the current global situation “calls for doing everything possible to avert a new arms race.”

The original START treaty, signed in 1991 between the US and the Soviet Union, had limited deployed nuclear warheads to 6,000 for each party. New START succeeded it, signed in Prague by the US and Russia, the Soviet Union’s successor state. Despite a temporary technical suspension three years ago, both countries were believed to be following the treaty.

The treaty’s expiration follows a pattern of eroding arms control frameworks. Previous agreements such as the Intermediate-Range Nuclear Forces Treaty, the Open Skies Treaty, and the Conventional Armed Forces in Europe Treaty have already collapsed, limiting oversight on nuclear and conventional forces.

Former UK Chief of Defence Staff, Admiral Sir Tony Radakin, warned that the collapse of these treaties represents “one of the most dangerous aspects of our current global security,” citing the increasing role of nuclear weapons.

Russia’s former President Dmitry Medvedev, who signed New START in 2010, said its expiration should “alarm everyone,” highlighting the risk as global powers modernize their nuclear arsenals. Meanwhile, US President Donald Trump has downplayed concerns, stating last month that “if it expires, it expires… We’ll just do a better agreement.”

Any future arms control deal is expected to consider other nuclear powers. Washington has suggested including China, while Moscow has proposed France and the UK. Analysts warn that rising strategic capabilities and new weapons development in the US, Russia, and China make reaching a new treaty more challenging.

Darya Dolzikova, senior research fellow at the UK-based RUSI, noted that both the US and Russia are modernizing their nuclear forces, including new long-range hypersonic missiles and advanced weapons systems, which could intensify global tensions and complicate future agreements.

The expiration of New START signals a potential new era of instability in nuclear arms control. Observers emphasize that urgent diplomatic efforts are needed to prevent an uncontrolled arms race and ensure global security.

With inputs from BBC