How can neutral countries participate in the affairs of warring countries?

During World War II, but the point remains true in many other war-time contexts, neutral countries participated in the affairs of formal belligerents in a number of ways. Diplomatically, 'neutrals' sometimes offered to serve as mediators or were asked to mediate between nations at war. On humanitarian grounds, neutral nations sometimes sent aid-teams or inspectors into warring nations. Further, trade and financial transactions were sometimes the basis for interaction between 'neutrals' and nations-at-war.