Final answer:
If the Dunkirk evacuation had failed, the British Expeditionary Force and many French troops would have been captured or killed, drastically undermining Britain's ability to defend itself and possibly altering the outcome of World War II. It would have impacted the morale and the geopolitical landscape post-war.
Step-by-step explanation:
If France and British troops did not escape Dunkirk to Britain, the immediate consequence would have been the loss of a significant portion of the Allied forces, amounting to over 300,000 soldiers.
In such a scenario, with nearly the entire British Expeditionary Force and significant numbers of French troops captured or killed, the ability of Britain to defend itself against a potential German invasion would have been severely compromised.
Additionally, without the evacuated troops, the morale of the British public and the army could have suffered a devastating blow, possibly changing the course of World War II. The 'Miracle of Dunkirk' not only saved a substantial number of troops but also served as a symbol of hope and resilience for the Allied forces.
Without this evacuation, Britain might not have been able to rebuild its military strength to continue fighting. The long-term effects could have included an earlier and more vigorous German focus on Britain, potentially altering the outcome of the war, and perhaps even influencing whether the United States, which at the time was still neutral, would have entered the conflict on the side of the Allies.
The geopolitical landscape of post-war Europe would likely have been very different had the Dunkirk evacuation not occurred.