A UNITED FRONT!

The United States officially declares its support for Chinese - primarily true Chinese, but also communist Chinese- efforts to stave off the Japanese invaders.
A declaration of war is made upon Japan, as the United States hereby joins the united front.
Through diplomacy, Mongolia has agreed to do so in similar fashion, albeit moreso for the support of communist China.
It is in the best interests of all Chinese factions or benefactors- both warlords and legitimate governments- to cease infighting and to take up arms against Japan.
Differences are set aside with these Asian communists due to the greater threat of the rising sun.
The American navy begins running interception on Japanese supply lines through conventional fleets and submarine attacks alike.
Aircraft, to the best of their ability, attempt to secure air superiority.
If superiority is won in a region, tactical bombing soon follows through tactical bombers, dive bombers, and heavy fighter aircraft.
The U.S. army begins deploying
en masse to the Republic of China, their intent being to move north to support efforts in liberating Manchuria and Korea.
Their naval convoys are escorted, of course.
The United States now considers itself to be in a state of war unlike the more one sided conflicts it was recently in, such as the support of nationalist Spain, and the quelling of Canadian rebellion.
As such, yellow journalism and general propaganda skyrocket.
They focus on the brutality of the imperial Japanese regime, and the twisted, fanatical honor system in Japanese warrior culture.