The sea around it.
In the past, the sea around Japan helped to protect them from invasion. However, after the USA developed the atom bomb, the difficulty of invading Japan influenced them to drop the atom bomb.
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During the early part of World War Two, the distance of Japan was so far from the US that the Pacific Ocean protected Japan from invasion. The Chinese had no chance to invade Japan and the USSR had an agreement not to engage in combat until the very last days before Japan's surrender.
Japan justified its invasion of Manchuria in 1931 through a combination of economic and security arguments. They claimed that the region was necessary for Japan's expansion to secure resources and markets for its growing population. Additionally, Japan asserted that it was acting to protect the lives and property of Japanese citizens in Manchuria, particularly following the Mukden Incident, which they portrayed as a pretext for military intervention. This invasion was framed as a civilizing mission to bring stability and order to a region they claimed was in chaos.
It was designed to protect Saudi Arabia from an invasion of Iraqi forces. It was designed to protect Saudi Arabia from an invasion of Iraq forces
They were completely overpowered both in men and firepower, so not much was done except a few outbreaks by the Manchurians against Japanese soldiers that were quickly subdued. They also called the League of Nations, who responded to their help by sending Lord Lytton, who wrote a report on Japan's invasion.
He wanted to avoid an invasion of Japan.
insect invasion
See "The Planned Invasion of Japan: Bibliography of works by D. M. Giangreco" at http://www.waszak.com/giangreco_bibliography.htm .
Japan invaded Manchuria in 1931 .
Australia was not invaded by Japan. One city was bombed. America used Australia as a base for the reconquest of the South Pacific.
it would prevent high casualties that would be caused by an invasion of mainland japan