By 1996, Ukraine voluntarily gave up all of its nuclear arms and acceded to the Nuclear Non-Proliferation Treaty. In exchange for making the world a safer place, it received security assurances from Britain, the United States and Russia in the form of the Budapest Memorandum, signed by Bill Clinton, Boris Yeltsin, and John Major, with pledges to “respect the independence and sovereignty and the existing borders of Ukraine” and the “refrain from the threat or use of force against the territorial integrity or political independence of Ukraine.”

Russia has since clearly and repeatedly violated Ukraine’s sovereignty and territorial integrity, initially with the Crimean land grab and, most recently, by its invasion of eastern Ukraine. As a result, Russian backed forces shot down Malaysian Airlines flight MH17, killing 298 innocent men, women and children. Last week, British journalists have spotted columns of Russian armored personnel carriers driving straight through the border into Ukraine. Russian soldiers and tanks are now being caught red handed fighting on sovereign Ukrainian soil.

For Vladimir Putin, Ukraine’s independence, which effectively broke up the Soviet Union, was a very bitter pill to swallow. Putin has publicly stated that the demise of the Soviet Union was “the greatest geopolitical catastrophe of the 20th century.” Russia’s authoritarian president does not want to accept Ukraine’s independence and his actions in eastern Ukraine are targeted at destabilizing a prosperous and independent state.

As Ukraine is now under fire from Russian military forces, the United Kingdom and the United States must step up and review the assurances that were given two decades ago. Global security and any future prospects of nuclear non-proliferation are at major risk if promises endorsed by US and UK are not adhered to. Barack Obama and David Cameron must stand up to Vladimir Putin and keep to the agreements made by their predecessors.

Andy Hunder is director of the Ukrainian Institute in London. The article is reprinted with the author’s permission and was originally published here.