A nation can guarantee a limited number of nations (I say three at max) if the nation whom they are guaranteeing is at peace. If someone declares war on the guaranteed nation: the person who guaranteed independence of that country gets to be dragged into the conflict. they get dragged into the conflict to help combat the aggressor. (Either by two wars defending the defender or by a joint war defending the defenders). The guarentee of independence is only for nations. The guarentee of independence can be revoked when military power reaches lower than that of whom you are defending or in revoked in peace time optionally.
I just thought it would be nice if you have a bond with a nation that you can express your longing to defend a fellow nation without having to do all the work of not being supportive enough to defend someone's nation with a more nation to nation option rather than alliance dependency for your nation being the only option for support that actually helps when your a major or highly concentrated target.