Why is a US security guarantee essential for Ukraine's defense against Russia?

According to the British security official, while they are prepared to commit British forces on the ground alongside other nations if there is a lasting peace agreement, a US backstop remains essential. The official emphasizes that a US security guarantee is the only effective way to deter Russia from attacking Ukraine. This stance highlights the continuing reliance of European defense strategy on American military capabilities despite efforts to increase European defense spending. As noted by the defense policy analyst, there is an urgent need for all NATO members to increase their defense budgets while maintaining the critical US deterrence component in their collective security framework.