I got this from Erwan Mahé‘s excellent newsletter Thaler’s Corner:

NOTE OF THE GOVERNMENT OF THE U.S.S.R.
DELIVERED TO THE POLISH AMBASSADOR IN MOSCOW
IN THE MORNING OF SEPTEMBER 17, 1939
September 17, 1939

Mr. Ambassador!

The Polish-German War has revealed the internal bankruptcy of the Polish State. During the course of ten days’ hostilities Poland has lost all her industrial areas and cultural centres. Warsaw, as the capital of Poland, no longer exists. The Polish Government has disintegrated, and no longer shows any sign of life. This means that the Polish State and its Government have, in point of fact, ceased to exist. In the same way, the Agreements concluded between the U.S.S.R. and Poland have ceased to operate. Left to her own devices and bereft of leadership, Poland has become a suitable field for all manner of hazards and surprises, which may constitute a threat to the U.S.S.R. For these reasons the Soviet Government, who have hitherto been neutral, cannot any longer preserve a neutral attitude towards these facts.

The Soviet Government also cannot view with indifference the fact that the kindred Ukrainian and White Russian people, who live on Polish territory and who are at the mercy of fate, should be left defenceless.

In these circumstances, the Soviet Government have directed the High Command of the Red Army to order the troops to cross the frontier and to take under their protection the life and property of the population of Western Ukraine and Western White Russia.

At the same time the Soviet Government propose to take all measures to extricate the Polish people from the unfortunate war into which they were dragged by their unwise leaders, and enable them to live a peaceful life.

Accept, Mister Ambassador, the assurance of my high consideration.
People’s Commissar for Foreign Affairs of the U.S.S.R.
V. MOLOTOV

To
The Extraordinary and Plenipotentiary Ambassador of Poland, Mr. Grzybowski
Polish Embassy
Moscow