In his dream Yaakov Avinu saw angels going up the ladder that led to heaven and coming down again. The medrash offers various interpretations of this image. One of the understandings presented by the medrash is that the angels going up and down represented the historical rise and fall of various nations, with the angels representing the sar of those various nations.
The medrash continues to explain that Hakadosh Barcuh Hu told Yaakov Avinu, "now it is your turn to climb up the ladder to represent the success of the Jewish people." Yaakov was afraid to do so, for the angels of all the other nations ultimately went down again, representing the fall of all those nations, and he didn't want the Jewish people to fall. Whereupon Hakadosh Barcuh Hu told Yaakov not to fear; "I will be with you. I will hold your hand. The Jewish people will not disappear."
The entire existence of the Jewish people from its very outset was not natural. According to tradition, the avos and imahos were akorim; b'derech hateva none of us should exist. This is the simple meaning of the Talmudic statement (Shabbos 156), "ein mazal l'Yisroel." According to the Ramban, "mazel" is a reference to the natural rules of history. Jewish history is not subject to any of those rules. In Yaakov's fight with the malach, the malach succeeded in injuring Yaakov's leg, but Yaakov won the fight. The malach represents the laws of nature (as the Talmud tells us that every blade of grass has a malach causing it to grow) and Yaakov's victory over the malach represents the principle that Klal Yisroel is l'ma'alah min hateva.
If one were to draw a graph representing the history of any other nation or culture, the graph would go up, reach a peak, and then do gown, representing the rise and fall of that nation. But if one were to graph the history of the Jewish people, the graph would zigzag, i.e. have many alternating peaks and valleys. When we observe the mitzvos we rise, and when we sin we fall.
The navi (Malachi 3:6) tells us that just as Hashem is above teva, and therefore not subject to change, so too Bnai Yisroel are also above teva and will not disappear.