Why the Catastrophes?

We've just entered the Three Weeks again, the notorious period between the 17th of Tammuz and Tisha B'Av, when the worst things happened to the Jewish People throughout history. We can't understand the Three Weeks without understanding the root of Tisha B'Av and the destruction of both Holy Temples. It was the eve of Tisha B'Av when the twelve spies returned to the Israelite encampment from 40 days of scouting out the Land of Israel. All of them except for Calev ben Yephuneh from the tribe of Judah and Yehoshua Bin Nun from the tribe of Ephraim said slander about the Land of Israel and about how dangerous it was. That night, the entire Israelite nation sat in their tents and cried. Hashem said to them, "OK, you want to cry for nothing? I'll give you plenty of real reasons to cry for generations to come." What's the connection between the sin of the spies and everything that's happening to us today?