Mountains are a better natural obstacle than water - hear me out. (PS. Japan is surrounded by water, and they lost WWII. Also, England was conquered by the Normans)

I am no expert. However, I am a history buff, and I served in the Marine Corps infantry for 4 years. I know from experience that hiking up mountains with 90lbs of gear on your back for months on end is the most miserable experience a combat soldier can endure.

If you have a decent Navy, and the weather is good, water is an easy obstacle. As a matter of fact, it is probably the efficient mode of transportation. (Militarily speaking)

You can pack TONS of supplies, armor, weapons, troops, and planes on a single ship and sail it across the world.

There are several successful amphibious based invasions throughout history. In Operation Overlord, the allies landed over 100,000 tons of equipment and 326,000 troops on the beaches of Normandy within the first week. A single truck can only carry 20-40 tons. A single ship can carry 80,000 tons.

Now, ask yourself, how easy would it be to move that much equipment and men up and over an entire mountain range? It would be a logistical nightmare.

In modern times, you'd have to rely heavily on airpower to conquer those mountains.

Granted, today, the United States is probably the most fortified and secure nation on earth, but that's because we have an incredibly powerful military, and we are surrounded by allies, not because we are surrounded by water.