People always say that Alexander singlehandedly conquered the entire Persian empire. I was wondering if the Persians were much of a challenge to Alexander. As far as I can tell, Alexander's army was superior to all others during his time; the hoplite was considered the fiercest warrior before the Roman legionnaire. If the Persian empire was at its peak, would Alexander still been able to be so successful in his campaign?
To some degree, yes the Achaemenid Empire was not the indomitable force it had been 6th century. In the century or two leading up to Alexander's invasion, the Persian Empire was going through a period of instability. Egypt had staged rebellions beginning in 487 BCE and continued into 460 with Athenian aid. Eventually gaining independence in 400 BCE. It is important to note, however, that the Achaemenid Empire reconquered Egypt in 343 BCE. So they were no pushover by any measure.
Another example of instability can be found in the revolt of the satraps in 366-360 BCE. During this period the satraps (or governors) of Phrygia, Cappadocia, and Armenia led an armed revolt against King Artaxerxes II Mnemon. Eventually, this rebellion collapsed when the Satraps of Armenia sought reconciliation with King Artaxerxes. The willingness with which these satraps rebelled against the King shows the instability in the noble class.
Finally, during Cyrus the Younger's coup in 404 BCE Cyrus recruited roughly 10,000 Greek mercenaries to fight his aforementioned brother, Artaxerxes II. Not only does this dynastic struggle again underscore the insability in the Persian government, but the aftermath of this coup gives insight into chaos of 5th century Persia. Xenophon and his 10,000 mercenaries escaped by marching from Cunaxa in central Persia, to Trapezus on the black sea. This journey covered hundreds of miles and took weeks to complete, and yet they were never intercepted by a Persian force. Their ability to walk through a large portion of the Achaemenid Empire unopposed hints at the underlying instability at the time.
By the time Alexander crossed the Hellespont in 334 BCE the Achaemenid Empire was not in a state of collapse, but their ability to counter military threats was severely hindered by the political turmoil of the last two hundred years. It's impossible to say if Alexander would have been defeated had Persia been more stable, but they certainly would have posed an even greater challenge.
Sources:
Gershevitch, Ilya. The Cambridge history of Iran: The Median and Achaemenian periods
Rawlson, George. Ancient Egypt
Xenophon. Anabasis