Zechariah Ch. 9:8 says: “But I will encamp at my temple to guard it against marauding forces. Never again will an oppressor overrun my people, for now I am keeping watch.” But in reality, Jerusalem was soon overrun by the Persians, the Greeks, and the Romans, who utterly destroyed the second temple in 70 CE (roughly 586 years after it was completed). Does it mean Zechariah predicted false things?
This is a good question. Before answering your question, I will note that, for a Jew living in Judea in period of Persian or Greek control of the Promised Land, yours is a really good question. I am sure that the Jews found themselves asking this many times. I certainly can see why you find this hard to grasp. The same goes for the destruction of Jerusalem in AD 70.
But… We need to look at the entire prophecy. Here is Zechariah 9:8-10 (NIV): “But I will encamp at my temple to guard it against marauding forces. Never again will an oppressor overrun my people, for now I am keeping watch. Rejoice greatly, Daughter Zion! Shout, Daughter Jerusalem! See, your king comes to you, righteous and victorious, lowly and riding on a donkey, on a colt, the foal of a donkey. I will take away the chariots from Ephraim and the warhorses from Jerusalem, and the battle bow will be broken. He will proclaim peace to the nations, His rule will extend from sea to sea, and from the River to the ends of the earth.”
This is a messianic prophecy, as well as a kingdom prophecy. The king riding on a donkey is rather obviously messianic—applying to Jesus on the Sunday before his crucifixion. The description of a kingdom of peace that will extend over the earth is not about an earthly kingdom. It is a spiritual kingdom whose “realm” spreads over the whole earth today, and which will eventually overtake the earth. This is the same kingdom prophesied in Daniel 2:44-45 and 7:13-14, 26-27, as well as in Isaiah 2:1-5 and other kingdom prophecies.
I hope this makes sense.