Why is a US security guarantee essential for Ukraine according to the British Leader?

According to the British Security Official, a US security guarantee is essential because it's 'the only way to effectively deter Russia from attacking Ukraine again.' While the official expressed willingness to commit British forces on the ground alongside others if there is a lasting peace agreement, they emphasized that a US backstop is absolutely necessary. This stance highlights the crucial role of American military power in European security architecture and reflects the ongoing concern about Russian aggression in the region. The British position demonstrates both a readiness to contribute to multinational security efforts while acknowledging the irreplaceable deterrent effect that US military commitment provides against potential Russian threats.