The Seventy Years usually referred to as the seventy years captivity in Babylon, is scripturally styled the "seventy years desolation of the land." This desolation God had predicted by Jeremiah the Prophet thus: "And this whole land shall be a desolation, and…shall serve the king of Babylon seventy years." (Jer. 25:11) "Thus saith the Lord, that after seventy years be accomplished at Babylon, I will visit you, and perform my good word toward you in causing you to return to this place." (Jer. 29:10) In 2 Chron 36:17-21 the fulfillment of this prophecy is recorded. The reason why it was just seventy years and why the land was made completely desolate, is stated thus: "He brought upon them the king of the Chaldees [Nebuchadnezzar, King of Babylon]…and them that had escaped from the sword carried he away to Babylon, where they were servants to him and his sons until the reign of the kingdom of Persia: to fulfill the word of the Lord by the mouth of Jeremiah, until the land had enjoyed her Sabbaths; for AS LONG AS SHE LAY DESOLATE SHE KEPT SABBATH, to fulfill threescore and ten  years."
Here we see clearly marked the time when the seventy years began. It was at the time of the carrying away of Zedekiah, Israel’s last king, and not at the time of his father's captivity, eleven years previously. We see distinctly that it was not seventy years captivity, but seventy years desolation of the land. Apparently the land was not desolate during the eleven years of the reign of Zedekiah. This is an important point in history, and one which has misled many in their chronological reckonings.
Israel had failed to observe properly the Sabbatic years of which the Jubilees were the chief. It certainly was a severe test of obedience toward the Heavenly King. God had forewarned them through Moses that if they were disobedient to the laws, He would punish them for it. In the same chapter where He tells them of the punishment of seven times under Gentile rule, He also talks about the desolation. God tells them that if they would neglect the year Sabbaths, He would punish them for it by desolating their land. (As a matter of fact, the Seventy Years Desolation was also the beginning of the seven Gentile Times.) The scripture reads thus: "Your land shall be desolate and your cities waste. Then shall the land enjoy her Sabbaths, as long as it lieth desolate and ye be in your enemies' land…because it did not rest in your Sabbaths when ye dwelt upon it." Lev. 26:34, 35, 43
The Scriptures assure us that God foresaw the calamities which then came upon his chosen people. The lessons He had sent them in the nature of milder chastisements had availed little. The example of scattering the ten tribes on account of their idolatry had not been taken to heart by the two tribes. Now God would scatter them all. The land would then lie desolate for seventy years–a period in which those who loved idolatry might go their way. It was a period, too, in which those who still reverenced the Lord–as for instance Daniel and others—would look back longingly to the land of promise. They would read more attentively than ever the records of the Lord's dealings with their nation and see how He had predicted this very trouble. The captivity, which they now were experiencing, did lead them to a better condition of heart and to instruct their children in the right ways of the Lord.
At the close of the seventy years desolation, the Lord raised up Cyrus the King of Media and Persia. Cyrus conquered the Babylonians and offered liberty to all of the Jews who desired to return to their own land. That proclamation would have little influence upon the vast majority of the Israelites who had gone into captivity. Their idolatrous hearts had undoubtedly forgotten all about God and the covenant made with their fathers. They were fully assimilated with the heathen by intermarriage, etc. The indifferent Israelites, who had settled in the foreign land, realized that it would be to their disadvantage to leave their Babylonian homes to go back to Palestine. Consequently, of all the hosts of the twelve tribes that went down to Babylon in various captivities, only about 50,000 accepted the offer of Cyrus to return. Of these, only a few had ever seen Palestine; they had merely heard of it through their parents and the few aged survivors. These returning Jews, however, had learned well the lesson which their fathers refused to learn. From the day of the return from the Babylonian captivity, there has not been recorded any further idolatry in Jerusalem and Judea.