What challenges does NATO face regarding troops in Ukraine as part of a peace deal?

According to the US Defense Secretary, any NATO forces deployed to Ukraine would not be protected by NATO's principle of collective defense, creating significant vulnerability. This limitation exists alongside Russia's firm stance that the presence of NATO troops in Ukraine as part of any peace deal is 'completely unacceptable.' These constraints create a complex diplomatic and military challenge. For the UK specifically, the Prime Minister's proposal would require substantially expanding British military capabilities and potentially accepting the risk of direct military engagement with Russia. This situation illustrates the delicate balance between peacekeeping ambitions and geopolitical realities in the ongoing Ukraine conflict.