Yeshaya Perek 27
"וְהָיָה בַּיּוֹם הַהוּא, יִתָּקַע בְּשׁוֹפָר גָּדוֹל, וּבָאוּ הָאֹבְדִים בְּאֶרֶץ אַשּׁוּר, וְהַנִּדָּחִים בְּאֶרֶץ מִצְרָיִם; וְהִשְׁתַּחֲווּ לַיהוָה בְּהַר הַקֹּדֶשׁ, בִּירוּשָׁלִָם"
"And it shall be on that day, the great Shofar will be blown, and those lost in the Land of Ashur will come, and those pushed off in the land of Egypt (will come), and they will bow to Hashem on the holy mountain, Yerushalayim. (27:13)"
The passuk mentions that Jews will be exiled in, Ashur and Egypt, and assures us that those exiled there will return. Rav Tzadok HaKohen understood a deeper level of meaning behind the two places, Ashur and Egypt. Each land is representative of certain problematic behavior which people will adopt in exile, and the redemption refers to the correction of that behavior, and its channelling into Avodas Hashem.
Rav Tzadok explains that being "lost in the land of Ashur" means to be lost in the pursuit of pleasure (the root Ashur means happiness). The Navi is describing how in the exile, people will channel all their energy, attention, and drive to the pursuit of selfish goals. Those "being pushed off in the land of Egypt" represent not misdirected excitement and enthusiasm, but a lack of drive and effort in any shape or form. One of the proofs Rav Tzadok brings for this idea is that the Nile can provide water for the entire land of Egypt, unlike Eretz Yisrael which requires prayer, and great physical work, to irrigate the land. Therefore, the exile of Egypt is symbolic of a lack of energy, goals and drive.
Rav Tzadok continues this theme to bring out an incredible lesson. The passuk here describes the redemption from Ashur first, since those who have aims, drive, and energy, even if they are not for great things, will be redeemed before those who are lethargic. Energy and excitement can be channeled gradually into other things and so those from Ashur will be redeemed, i.e. find their correct path in Avodas Hashem, before those people who are lethargic. In a state of lethargy, there is no drive to work with and no energy to channel towards anything higher.
In becoming an Eved Hashem, one must first have a mindset of doing and striving for something. Even if initially one’s goals are not ideal, one can gradually alter his goals, and divert his energy into greater things.