Answer:
I would say the undeclared Anglo-Spanish war of (1585–1604). The Battle of Thermopylae (480 BC) , and The fall of Constantinople (1453) are the most significant and here's why :
Explanation:
In the Anglo-Spanish war , if the English lost, the Spanish would have likely landed an invasion force and succeeded in overthrowing Elizabeth I, thus obliterating the Anglican church. Rather than becoming the most formidable sea power in Europe and, thus, being able to project that sea power, England would have become a Spanish vassal, making it much easier for Spain to “rub out” Protestantism in the rest of Europe. Furthermore, the notion of “Great Britain” would be strangled in its bassinet.
In the Battle of Thermopylae had not the Spartans, Thespians, and Thebans held the pass and allowed the other Greek forces the ability to muster a defense against the invading Persians it is entirely likely that Alexander the Great would never have had Aristotle as his tutor, and never have spread Hellenization to the known world resulting in the rise of classical culture and the offshoots it created.
In the Fall of Constantinople ; essentially this battle played a huge role in the formation of the modern world. as places like the Americas would never be discovered and the world maybe would be a different place, if the battle of Constantinople didn't happen.
It led to Japanese expansion into China--during the 1930s, Japan expanded their territory into mineral rich areas of Manchuria and then continued to invade China.
The goal of this expansion was to become a dominant world power as well as secure markets and resources the small island nation needed to become an industrial power.
