Throughout history, armies have used different strategies and weapons to prevent adversarial forces from occupying or traversing important territory. The concept of the strategy is known as Anti-Access/Area Denial (A2/AD). Most individuals would know this strategy from their knowledge on land mines. The modern landmine evolved from the anti-personnel weapon, the caltrop. The caltrop is an ancient anti-personnel weapon (although still used on modern battlefields), consisting of two or more sharp spines, arranged in a way that these spines always point upward. One of the earliest noted uses of the caltrop on the battlefield was in 331 BC, when Darius III deployed these weapons against Alexander the Great during the Battle of Gaugamela.
The A2/AD strategy was also instrumental in land and naval battles. When the Persian’s invaded Greece, Xerxes I brought with him nearly one million troops. The Greeks could only field 125,000 soldiers to counter the massive Persian invasion. One of the more famous and celebrated A2/AD battles during this conflict is the Battle of Thermopylae. The Greek city-states, led by King Leonidas I and his 300 Spartans, assembled a defensive force of 7,000 soldiers to counter the Persians. While Herodotus states the Persian’s assembled an army of 2.5 million soldiers, modern scholars believe that number to be closer to 70,000. Even with 70,000 soldiers, the Persians outnumbered the Greek’s ten to one. While the Persian’s triumphed in the battle, the Greek’s were able to stall and inflict crucial casualties on Xerxes army. The Greeks were able to defeat the Persians in the naval Battle of Salamis, and the Battle of Plataea which ended the Persian invasion of Greece.
When the Spanish launched their massive armada to invade England, they were met with a small, but crucial naval forces to protect the English coast. The English naval forces were able to keep the armada from supporting the Spanish troops in Flanders, forcing the Spanish to call off the invasion and sail back to Spain. The armada sailed around the British Isles, but met severe storms in the North Atlantic that inflicted far more casualties than the English navy. Of the 135 ships that left Spain in the armada, over a third of those vessels never returned home.
During World War II, the development of radar became an important A2/AD resource. Following the victory over France and the Low Countries, Germany looked toward Great Britain to finish their conquest of Western Europe. The Germans set plans for the invasion, known as Operation Sealion. Before launching an amphibious invasion, the Germans wanted to gain air superiority over the Royal Air Force. The Germans launched a massive air campaign that became known as the Battle of Britain. The development of radar helped the RAF Fighter Command locate German aircraft and scramble British fighters to counter the threat. In the end, the RAF defeated the Germans, forever stalling their planned invasion of the United Kingdom.