I recently had a conversation with a friend who was making what seemed like spurious claims about the Japanese during WWII. He seemed so certain that he was right that I brought it here. He claimed the following: The US navy was smaller than the Japanese navy and therefore the US was under threat of siege and thus the dropping of the atomic bombs was justified. He even went so far as to suggest that the Japanese would have taken over significant parts of the world (including the US) and held onto them for a long time. He induced the idea that we perhaps would have been conversing in Japanese had events panned out differently. I strongly disagreed. I tend to disagree with any argument that tends towards justifying the atomic bomb, so I want to know if any of this true or not.
I'm afraid I may be looking at this out of context and I don't want to suppose that at this time during WWII the US navy was anywhere near the level of what it is today being the world's largest military superpower (ever). I do know that the US military was mainly built up in the years after the war and going into the Cold War. To compare more recent events that I'm more familiar with - I obviously don't buy the justification of the war in Iraq on the basis that we were being threatened. But could it have been that the US was significantly threatened at that time in WWII?
I suppose my question is this: Was it possible for Japan to invade the US in the later stages of WWII and/or was it even in their interest to do so? The purpose of the attack on Pearl Harbor was to weaken the US navy IIRC and certainly not about seizing territory. That brings me to believe that Japan had no interest in attacking US territory. I am, however, conflicted then with the dropping of the bombs on Nagasaki and Hiroshima. I have heard others argue that this was to defend the US rather than to deter and cripple the Japanese politically and economically (which is what I believe).
I'll tell you what the best history professor I've ever had told me. The moment Franklin D. Roosevelt signed the declaration of war against the Empire of Japan, Japan had lost the war.
The fact of the matter is that yes, the Japanese navy was superior to that of the American navy at the start of the war, especially thanks to Pearl Harbor. Something very few people realized was how much the carrier changed the face of naval warfare. Before the carrier, naval battles consisted of ships bombarding each other directly. After the carrier, naval battles were now two fleets defending themselves from the enemies aerial assault. This was extremely important, because the Japanese Zero was the best fighter plane in the world at the time (it largely sacrificed the safety of the pilot for this superiority however). However by the end of the war American fighters, such as the Hellcat, had overtaken them in technological superiority.
So your friend is right about one thing, the US navy wasn't able to stand up to the Japanese navy at the beginning of the war. This was all turned around by a decisive victory at the Battle of Midway in which we sunk four Japanese carriers. The Japanese had sunk many of our own ships, but the difference is, and this is perhaps the single most important factor in deciding who won the war, American industrial capacity. When Japanese ships sunk, they could not hope to replace them at the rate at which we were doing so. In other words, the true secret to American victory in WWII was it's ability to produce the implements of war. Both Germany and Japan were critically flawed in this respect.
The Japanese actually understood this, and most knew that total war with the American's would end in failure, even with a decisive strike at Pearl Harbor. Attacking Pearl Harbor was not just to weaken the American fleet. The hope of such a destructive attack was the strike at American resolve. Unfortunately for them, their notion that American resolve was so fragile stemmed from racism, which history will tell you again and again is not just wrong, it is stupid. The Japanese saw the West, and America especially, as a decadent, prissy society. They believed themselves superior, and they believed we would not fight. The hope was that by destroying the American Navy at Pearl Harbor, the Americans would piss in their boots and then agree to give the Japanese free reign of the Pacific. There is no evidence whatsoever that they intended to move on from Pearl Harbor onto the American mainland, as this would have been forcing the Americans to fight, which would have been blatant suicide.
So in short, your friend is wrong that the Japanese were on track to dominate the world. By 1945 when we dropped the atomic bomb they had one million troops still bogged down in China, they had been stopped by the United Kingdom in Burma, and they had been forced out of nearly all of their Pacific holdings, culminating with Okinawa in June of that year. When we dropped the atomic bomb the Japanese had virtually no chance of winning the war. Dropping the bomb had nothing to do with stopping Japan from winning, it had everything to do with getting Japan to finally surrender unconditionally.
If you want, I can go into more detail about the atomic bomb and it's justification, since I know good deal about that subject as well. If you're interested in learning more (sources) I recommenced The Rising Sun by John Toland as well as The Battle of Okinawa by George Feifer. The first is a more general overview of Japan and it's doctrine going into the war and the events leading to it's eventual surrender. The second covers Japan specifically near the end of the war, and discussing the justification for the atomic bomb as well.