In tactical terms? It prevented the occupation of Persia by the Axis, allowing the Persian Corridor that was instrumental for the US supplies to the Soviet Union and eventually the success in the Eastern Front and the control of the oil fields.
Jus ad bellum as a concept can be even more vague than words like easily and it's a term used by 20th century politologysts to express their own particular opinions. The Just War as defined by Cicero, Saint Augustin and the Catholic Cathecisms don't demand the strict idea of last resort for military operations as long as intent, cause, authority and proportionality were included. The alternative not used was to allow the Axis to invade Persia.
ETA:
Quotation from FDR's letter to the Shah:
"Viewing the question in its entirety involves not only the vital questions to which Your Imperial Majesty refers, but other basic considerations arising from Hitler's ambition of world conquest. It is certain that movements of conquest by Germany will continue and will extend beyond Europe to Asia, Africa, and even to the Americas, unless they are stopped by military force. It is equally certain that those countries which desire to maintain their independence must engage in a great common effort if they are not to be engulfed one by one as has already happened to a large number of countries in Europe. In recognition of these truths, the Government and people of the United States of America, as is well known, are not only building up the defenses of this country with all possible speed, but they have also entered upon a very extensive program of material assistance to those countries which are actively engaged in resisting German ambition for world domination."