Das hing mit dem polnisch britischen Beistandspakt zusammen.

Agreement of Mutual Assistance between the United Kingdom and Poland-London (1939) - Wikisource, the free online library

Secret Protocol attached to the Agreement of Mutual Assistance between the United Kingdom and Poland signed on the 25th August 1939

The Government of the United Kingdom and Northern Ireland and the Polish Government are agreed upon the following interpretation of the Agreement of Mutual Assistance signed this day as alone authentic and binding.

1. (a) By the expression "a European Power" employed in the Agreement is to be understood Germany. (b) In the event of action within the meaning of Article 1 or 2 of the Agreement by a European Power other than Germany, the Contracting Parties will consult together on the measures to be taken in common.

Die Briten mussten somit vertragsgemäss der Sowjetunion nicht den Krieg erklären.