"And God said unto Avram: 'Go for yourself from your land, from your relatives and from your father's house to the land I will show you.'" "So Avram went has God had spoken to him, and Lot went with him, and Avram was seventy five years old when he left Haran." (Bereshis 12:1,4)
"And He said until Avram: 'Know for sure that your offspring will be foreigners in a land not their own and they will serve them, and they will oppress them for four hundred yeears." (Bereshis 15:13)
There is a well known principle in the Torah that events recorded within it occured in the ordered writen unless there is obvious evidence that they did. For example, the final parts of parshas Mishpatim (Shmos 23:20-24:18) are obviously out of place referring as they do to promises and events leading up to the giving of the Ten Commandments back in parshas Yisro. Other commentators dispute when the mutiny of Korach occured - soon after the Golden Calf or after the incident of the Spies where the story actually falls in the Torah.
What's interesting is that in this parashah an event falls out of place but unless one is paying attention, one misses it entirely. Consider the two verses at the beginning of this post. As read, Lech Lecha begins with God commanding Avraham Avinu to leave Haran. He then goes to the land of Canaan, Egypt and back to Canaan, separates from Lot and then winds up having to rescue him from four powerful warlords. After this battle, God promisees him a great future at the Bris ben HaB'sarim (Covenenant between the Pieces).
Except that's not how it happened if you look closely. The first clue is a verse not in Bereshis but in Shmos:
"The time of dwelling of the Children of Israel during which they lived in Egypt was four hundred and thirty years." (12:40)
Now, as Rashi easily proves in his commentary, our ancestors were not in Egypt for 430 years, but rather for 210 years (still a very long time to be slogging bricks around for no pay or drug plan!). Most commentators therefore refer this 430 back to the Bris ben HaB'sarim. But here's the problem. The number 400 is used during that disucssion. The commentators explain the 30 year difference by saying that it was 430 years from the Bris ben HaB'sarim until the Exodus from Egypt. So what's the 400 mean? They explain it to mean that this convenantal time line began when Yitzchak Avinu was born since he never enjoyed permanent resident status in Canaan like his father did.
But how old was Avraham when Yitzchak was born? Again, an easy answer: 100 years old.
So do the math: If Avraham was 75 when he went through the Bris ben HaB'sarim then Yitzchak was due to be born in 25 years which means that the verse in Shmos should have said 425 years. The only wayto reconcile this is to accept that Avraham was 70 years old at the Bris ben HaB'sarim which means this happened a full five years before the events recorded at the beginning of the parashah.
Why is it so important to know this? Look back at the beginning of Lech Lecha. What does God tell Avraham? Essentially to buy an open train ticket to... somewhere. Yet we don't read about how Avraham wandered across the face of the Earth checking out land after land and only choosing Canaan when God finally revealed Himself to him there. Neither do the words of Chazal suggest he did any such thing. He packed himself and his family up and went straight to Canaan. But how did he know to go there, of all place?
Our calculations give us the answer. His first choice of places to travel to was a land he had already been to five years earlier where he had already experienced a Divine revelation. Instead of trying to find God somewhere else he went with a place that had already revealed itself to be fitting for God to reveal Himself. And it was in Canaan that God wanted Avraham to settle and begin the process of creating our nation.
An interesting side thought occurs at this point. This may explain why Avraham knew to go to Canaan when he was 75 but what about when he was 70? A thought from the commentary of Rav Shimshom Rafael Hirsch may provide a clue. When Avraham arrives in the land, we are told "the Canaanite was then in the land" (Bereshis 4:6) which Rashi explains as their ongoing conquest of the area from the descendents of Shem. (Remember that Malchi-tzedek, the king of Shalem mentioned in 14:18 is generally held to be Shem and therefore the leader of all the monotheists in the world at the time) Could it be that five years earlier the invasion had not yet began and Canaan was the last place in the world where people who believed in and consulted God could still be found? This would make it the natural place for Avraham, who was searching for God, to go.
This leads to a final interesting point: Despite the fact that the land had been conquered by the Canaanites, Avraham still returned there five years later (at the beginning of our parashah) and still received a revelation from God. The presence of idol worshippers and their hegemony over the land did not change God's plans or His ability to carry out His plans to establish our people. How much more so in our own day can we look at the Land and State of Israel and see that despite the widespread abandonment of Torah and the many evils which infect our society there, there is still hope and potential for those who wish to spread the true word of God throughout the land. Most importantly, nothing in Israel now can prevent the revelation of God's will if we choose to seek Him out with all our hearts and souls. All the potential is before us. We need but see the opportunity and grasp it.