In truth, under normal circumstances, it was basically impossible for a large-scale mobilization of a country's army to be concealed from other countries.
Mobilizing an army was not as easy as one might imagine, and completely concealing the news was even more impossible. Even though satellites were not yet available for intelligence gathering, other countries could still detect such a large-scale troop movement very quickly.
For example, during World War I, both Germany and France were basically aware of each other's troop movements. Although they did not know exactly where the opposing army had been deployed, they could still make certain guesses based on the situation at the time.
In theory, the German Army's movements could not be concealed from the French people, and the French naturally understood Germany's true intentions.
However, one factor was also very important: the Germans had just peacefully regained the Polish Corridor that they had once ceded through negotiations.
Sending troops to retake control of one's own territory appeared perfectly normal. That was exactly what the French thought. They paid little attention to the German Army's movements because there was not much activity along the Franco-German border.
To ensure that their offensive had no threats at their backs, the Germans decided to resolve the issues of Spain and Australasia before launching their attack.
After a meeting of the German-Italian-Island Nation Triple Alliance, Germany and Italy increased their support for the Spanish Nationalist Army and launched a final offensive against Madrid, which was still under the control of the Spanish Government.
Since the outbreak of the war, the Spanish Civil War had already become a large-scale war in the truest sense.
The combined number of government troops and Nationalist troops on the Spanish mainland alone had already come close to one million.
For Spain, a country