Reasons why it's pretty hard not to conclude that he was talking about the destruction of Jerusalem.
1. He told them that not one stone would be left standing.
Jesus left the temple and was walking away when his disciples came up to him to call his attention to its buildings. 2 “Do you see all these things?” he asked. “Truly I tell you, not one stone here will be left on another; every one will be thrown down.”
So he's talking about the destruction of the very temple they were admiring to him. That's our context.
3 As Jesus was sitting on the Mount of Olives, the disciples came to him privately. “Tell us,” they said, “when will this happen, and what will be the sign of your coming and of the end of the age?”
2. They're not asking about the end of the world. Jesus doesn't answer their question by changing the subject. The 'end of the age' is not the same thing as the end of the world. They know all about the resurrection on the 'last day.'
3. Jerusalem was destroyed and was surrounded by armies exactly as Jesus prophesied. The Christians of that time escaped because they were all warned by this prophecy and they left exactly when Vespasian's Army first encircled Jerusalem. Vespasian left, and Titus returned. Nobody got out after that and Jerusalem was made utterly desolate. Over a million Jews died and those who escaped holed up in Masada and then committed mass suicide. At the destruction of the temple, the Romans tore down the stones to the foundations because of the gold which had melted and run into the seams between the stones.
4. Jesus said that everything, all of what he had said, including his coming on the clouds would happen before that generation passed.
So everything in this passage points to the destruction of Jerusalem, the context of the question, the timing and the historical record. It's up to us to accept it as written, not whistle past the details, believing what some men say about this passage.
