Short answers, pulled from the story.
France declared war on Germany at exactly five o'clock in the afternoon on the 2nd of September 1939. The expiration of an ultimatum demanding German withdrawal from Polish territory marked this moment.
A defensive treaty with Poland served as the legal foundation for France's entry into conflict. This agreement obligated Paris to intervene if Berlin attacked Warsaw, which occurred on the 1st of September 1939.
The front page of Paris-soir displayed the news announcing the state of war on the 3rd of September 1939. Other papers followed suit with similar stark reporting that reached millions of households within hours.
London issued its own ultimatum earlier on the same day and acted in near unison with Paris. Both nations moved together into the first year of global conflict to honor their treaties and prevent Germany from exploiting any delay.
French forces began mobilizing immediately after the formal declaration on the 2nd of September 1939. Soldiers received orders to report to their units by early September while troops moved toward the Rhine River region in late August and early September.