Why is a US security guarantee considered essential in deterring Russian aggression against Ukraine?

According to British security officials, a US security backstop is the only effective way to deter Russia from attacking Ukraine again. While European nations are prepared to commit ground forces alongside others if there is a lasting peace agreement, they emphasize that American security guarantees provide the critical deterrent effect needed against Russian aggression. Simultaneously, European defense experts stress that all NATO members must increase their defense spending as Russia poses a threat not only to Ukraine but to all European nations. This dual approach of maintaining US security guarantees while scaling up European defense capabilities is seen as essential for creating a credible deterrent against future Russian military actions.