All countries accepted, de facto, the USSR's declaration of neutrality.
That is to say: all, including the belligerent Polish allies France and England, agreed that the USSR was not a belligerent power, was not participating in the war. In effect they accepted the USSR's claim that it was neutral in the conflict.
For the USA, see
A. New York Times September 18, 1939, p. 1:
"Moves by Soviet Give Roosevelt 'War Act' Question. He Is Faced With Decision Whether Neutrality Law Needs New Application."
It was understood here that the note sent by the Soviet to the powers proclaimed its neutrality and asserted that its expedition into Poland was to protect minorities where there was no remaining government. How this profession of neutrality will be received in official Washington remains to be seen.
Officials will study the situation and decide whether the Russian step falls within the
scope of neutrality of if it is in act of war. If it is decided that it is an act of war
the President may, if he chooses, declare that war exists and the Soviet would be placed
in the classification of a warring nation, just as Germany, Britain, France and Poland
have been designated.
B. New York Times September 23, 1939, p.3.
"Puzzle Increased in Russia's Status. State Department Lacks an Answer Whether or Not She Is Ally of Germany. 'Neutrality' A Paradox. Officials Without Precedent for Ruling Where Professed Non-Belligerent Takes Territory."
WASHINGTON, Sept. 22 - Whether or not Soviet Russia, although a professed neutral, is actually an ally of Germany was a question the State Department felt unable definitely to answer today.
For the French:
C. New York Times September 24, p. 69:
|Though both Mr. Chamberlain and M. Daladier criticized Russia's action, their governments made no apparent moves to draw the Soviet Union farther into the conflict. Presumably under the treaty of alliance with Poland, Britain and France would go to war with any nation that attacked the Polish State. But that move seemed unlikely. The entering of formal protest was expected to satisfy now that the republic, for the moment at least, had disappeared.|