This is how Churchill expressed inability to predict what Soviet Union will at the start of World War 2. The timeline below show why he could be puzzled:
a) August 23, 1939 - Soviet Union and Germany sign non-aggression pact
b) September 1, 1939 - Germany invades Poland (then ally of Britain and France).
c) September 3, 1939 - Britain declares war on Germany as a consequence of German's invasion of Poland.
d) September 17, 1939 - Soviet Union invades eastern Poland. Poland is defeated and partitioned between Germany and Soviet Union.
Therefore from Churchill standpoint: Soviet Union is allied with Germany, whom are at war with Britain. Go figure what Stalin is going to do?