Joe Biden’s belated decision to allow Ukraine to use US-supplied, long-range missiles inside Russia will be cheered by Kyiv and will almost certainly prompt the UK to follow suit.
But the stunning shift in US policy – just weeks before Donald Trump takes over as US president – will also trigger fury and fresh threats from Moscow at a time of increasing uncertainty about the future course of its war.
President Vladimir Putin has warned the West they would be playing with fire if they allowed Ukrainian forces to launch Western-supplied cruise and ballistic missiles at Russia, saying it could even trigger a global conflict.
British and US officials, though, have repeatedly advised their respective capitals not to be intimidated by Moscow’s sabre rattling.
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Yet there has been hesitancy, particularly in Washington, over the unleashing of US Army Tactical Missile System (ATACMS) ballistic missiles beyond the borders of Ukraine.
However, it seems that a move by North Korea to send thousands of its troops to fight with Russia has changed US calculations.
The New York Times, which was among the US media organisations to break news of the Biden administration’s decision on long-range missiles, reported that the first time the American weapons will be used inside Russia will likely be against Russian and North Korean troops battling a Ukrainian incursion in the Russian region of Kursk.
While a significant step up in support for Ukraine, the ability to use American long-range missiles inside Russia is not a war-winning development.
But it does expand Kyiv’s capacity to hit important military targets deliberately positioned by Russian commanders far back from the frontline. This includes stockpiles of missiles, drones and other ammunition used to strike Ukraine.
Just as important as the military impact, though, is the political signal that the US decision sends to the Kremlin about Washington’s willingness to defy Russian warnings about dire consequences should Mr Biden dare to grant Ukraine the permission it’s so long been seeking.
The dramatic move by the United States comes after months of lobbying by Ukraine’s leader, Volodymyr Zelenskyy.
A key thing to watch now will be how Mr Putin reacts the first time an American missile kills Russian soldiers on Russian soil.
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Moscow does not want a direct war with the United States and NATO – nuclear-armed forces with far greater combined firepower – but the Kremlin could well ramp up an already heightened campaign of sabotage and other forms of hybrid warfare across Europe.
Another important reaction to track will be how Mr Trump responds to Mr Biden’s move and whether he continues to allow Ukraine this permission once he takes over as the US commander-in-chief.
The president-elect has vowed to end Russia’s war in Ukraine quickly but he has not said how. Yet he has voiced fierce opposition to the continued gifting of vast quantities of American weapons to the Ukrainian military.