Actually, according to common sense, it is basically impossible to hide the large-scale mobilization of a country's military from other nations.
Mobilizing the military is not as easy as one might imagine, and to completely conceal such information is even more impossible. Even though there are no satellites to spy with intelligence, such large-scale military movements can still be quickly noticed by other countries.
For example, during World War I, the troop movements of Germany and the French Army were basically clear to both parties. Although it was unclear exactly where the opposing troops had been moved to, a certain level of guesswork could still be made based on the existing situation.
Logically speaking, the movement of the German Army could not be hidden from the French, and naturally, the French would also understand the true intentions behind the Germans' actions.
