Trust isn't a binary. If everybody trusted all of their neighbors as much as South Korea trusts North Korea then the world economy would collapse and we'd likely have billions of deaths.
It isn't binary but clever countries with a strong defense policy like the US and Switzerland also have insurance polices and backup plans for the case their allies turn against them since it's better to be a warrior in a garden than a gardener in a war. Guess in which case Europe is a the moment?