Pelranius wrote:
So what would the Japanese do when the US imposes its embargo on them? Simply roll over and go back home? In addition to political considerations, I don't think that the US felt that having Japan controlling China was a very safe strategic option. (Granted, that's assuming they could control China and even if they did accomplish that, that they wouldn't end up like Chipan, but FDR really doesn't have any way of figuring that out).
A major faction in the Japanese civilian government and some military leadership simply wanted to build Japans way out of the embargo. In oil particular would be solved by massive coal-oil conversion plants as the Germans made very considerable progress with. Since Japan had about a full year of war worth of oil stockpiled, and small existing synthetic oil industry. So careful rationing and restrictions on combat operations in China could have gotten them by for a while longer then that. Japan mainly needed oil for its navy and air power, not the army which had almost no trucks. So the navy would have to lay up a lot of warships (which it in fact did anyway, a lot of Jap cruisers were kept laid up at times to save manning costs), and the China war gets less air support. But the Army can keep fighting.
This plan it was argued would cost less then even a victorious war, and while it would take years it would also take years to rehabilitate and begin exploiting the resources of south Asia. Instead the war was launched with some really bullshit projections of how fast said resources would be exploited. It was expected in fact that Japan would not only restore the oil fields to full capacity within one year, but that they would exceeded prewar production! Instead they never ever got back to the prewar level, and couldn't even ship home what they could produce for lack of tankers.
Withdrawing from China was an option in its own right though. The US explicitly did not demand a withdraw from the Manchurian puppet state which had most of the resources Japan needed, just central China. By 1939 Japans leadership already knew China was a nightmare of hopeless fighting that would take many years to win. So this idea could not have been totally off the table, just absurdly unlikely. Keep in mind too that it really wasn't until after WW2 in Europe broke out that Japan went all out to try defeat China and occupied all the coastal ports. That move is what caused the US to launch its embargo and then later total asset freeze. Without war in Europe Japan may not feel bold enough to do this, and the war may be more contained from the onset and provoke a slower US reaction. Prior to the move to seize all the ports the fighting was massive, but still only involved a fraction of Chinese soil, and did not threaten a sweep to the south the way invading Hainan and Indochina did.
As it was IJA wouldn't even commit most of its divisions to central China, it kept them in Manchuria, because it knew they'd just be decimated after a few months and leave them weak against the Soviets. Instead a lot of the fighting was sustained by creating new units of raw conscripts, who in turn suffered yet heavier losses.
As for liner conversions, they might be used but they suck for the same reason USS Ranger did. No protection at all. So they run a high chance of burning and exploding from a single good hit.
"This cult of special forces is as sensible as to form a Royal Corps of Tree Climbers and say that no soldier who does not wear its green hat with a bunch of oak leaves stuck in it should be expected to climb a tree"
— Field Marshal William Slim 1956