a.    express/implied authorization from Congress + Presidential action = valid (b/c btwn Pres & Congress = 100% of foreign affairs powers)  (Dames Moore)
b.    Pres acts in contravention of Congressional authority (Congress said “no”) => presumption that Pres’s act was invalid (Youngstown)
c.    Pres acts in absence of Congressional authority, but not in contravention:  Pres and Congress may have concurrent authority:
i.    look to history (has this been done in past)
ii.    has Congress impliedly acquiesced in the past