What was the first unit to ever occupy Japanese soil?

The Fourth Marine Regiment was indeed the first unit to occupy Japanese soil. This significant historical moment occurred in 1853 during Commodore Matthew Perry's expedition to Japan, which aimed to open up Japan to the West after over two centuries of isolation. The presence of the Fourth Marine Regiment was crucial as it marked the U.S. military's role in establishing relations with Japan, leading to the Treaty of Kanagawa in 1854, which opened up two ports for American ships and initiated diplomatic relations.

Understanding this context highlights the importance of the Fourth Marine Regiment's role in U.S.-Japan relations, affirming why this answer is correct in the realm of military history and diplomacy.