“MacArthur said, “I shall return”. To be grammatically correct he should have said, “I will return”.”
MacArthur displayed a keen sense of etymology: “shall” comes from a Germanic/Old English word that, among other things, connotes “destined to”.
In this case, MacArthur said exactly what he meant.
Therefore, he was doubly correct: grammatically and historically.
Many were delighted when he did return.