By Sean Bell, military analyst
When the UK provided long-range Storm Shadow missiles to Ukraine, it was done on the understanding that these would only be used to target Russian forces in occupied territory, and not against Russia itself.
Despite the limitations, Ukraine has complied, and that appears to have improved US confidence to the point they are prepared to supply the ATACMS (Army Tactical Missile System) missile.
Indeed, we now understand that the US first supplied the ATACMS last month - in secret - and it appears to have proven its worth with attacks on a Russian-occupied airfield in Crime on 17 April, and earlier this week in an attack on Russian troop concentrations.
But will this new weapon risk an escalation of the war?
Russia knows that it faces strategic failure if the West were to commit fully to the defence of Ukraine, so it will continue to use threatening rhetoric and sabre-rattling to undermine Western political resolve.
The interesting question is whether the introduction of ATACMS will change the course of the war. Long-range missiles are very effective at targeting and influencing the "deep battle" - the logistics that feed the front-line fight. However, they have limited impact on the frontline.
The shortage of Ukrainian weapons is being exploited by Russia, which is making significant battlefield gains.
Volodymyr Zelenskyy has identified that his immediate priority is to "stabilise" the frontlines.
Instead, ATACMS and the new package of Western military support provides Ukraine with a lifeline.