Why did Argentina invade Falkland?

The Argentines believed that the islands had belonged to the Spanish empire and therefore belong to them. It is also theorized that there is a "tap into" the antarctic oil reserve ( which nobody can drill for because of the antarctic treaty) near or on the islands, bypassing the antarctic treaty meaning the islands are a bit more important than believed. This theory may be another motive for invasion and also would be a good reason why Britain defended it besides national pride.