Context:

Just days after the fall of Paris, General Charles de Gaulle, In one of the most important speeches in French history, told his fellow countrymen:

'Believe me...nothing is lost for France. The same means that over came us can bring us victory one day. For France is not alone! She is not alone! She is not alone! She has a vast empire behind her.She can align with the British Empire that holds the sea and continues the fight...

Question: Why do we use feminine pronoun for countries? Is it mandatory in English to use feminine pronoun for all countries and states or in some cases masculine pronoun"he" can also be used?