Jesus wants to discourage a false sense that these things will happen immediately. Lots of confusing and troubling and difficult tings will happen in the mean time, before the temple stone are thrown down. Here is the first exhortation. Watch out, or be careful, or take heed that no one deceives you. There is a grave danger of deception by false prophets who will misread the events.
People are always extra ready to be lad astray in times of trouble or uncertainty.
Men will emerge in the crisis leading up to the temple's destruction who will falsely claim the name and authority of messiah, and deceive many. In fact, as I tried to show in the letter, between Jesus death and the destruction of the temple in 70 ad there were a whole string of messianic pretenders who rose up and gained a flowing for a while. Most of them ended up crucified. When you hear of wars and rumors of wars, do not be alarmed. Here is the second imperative.