Hey there!
When we say that Athens was the cultural center of arts, mathematics, sculpture, literature, and pretty much everything else, it had a lot of influence on the rest of Greece. And when it went downhill, so did everything else.
King Philip the Second was a Macedonian King. When he took power, he made sure he would take over Greece- and he sure did. When he invaded Greece, he did it at a strategic time and that allowed him to gain control of the region. This was right after the greatest superpowers (Sparta and Athens) ever known in the Mediterranean region clashed with intent to beat each other and abolish them until the very end. This resulted in the weakening of the city-states in the area, and when Sparta finally won- it wasn't much of a victory when Philip's army came.
Philip used improved Greek strategies for battle. He knew what the Greeks were capable of- after all they had just united together and defeated the mighty Persian empire outnumbered more than 10 to 1. He used strategies such as the phalanx and eventually took over Athens and the rest of Greece. However, Sparta remained independent. This wasn't much of a surprise- being a warrior cult, Sparta wasn't necessarily willing to give up everything they'd ever worked for.
In short, the timing of Philip and his battle strategies greatly helped him take control of the Greek city-states - eventually leaving them for his son Alexander the Great to finish the job and take over the entire region, including the Persian Empire.
Hope this helps!