The military history of Iraq, due to a rich archaeological record, is one of the longest in written human history. The region of Iraq, which used to be Mesopotamia, has been referred to as the "cradle of civilization", and wars of conquest have been recorded in this region as far back as the third millennium BC. The area possesses strategic value, initially for the rich, fertile agricultural region in the Mesopotamian plain, and more recently for large petroleum deposits and access to the oil-rich Persian Gulf. The present territory of Iraq lacks significant strategic barriers, making it difficult to defend against foreign invasion.