Why did the three tribes that comprised Degel Machaneh Dan, (Dan, Asher and Naftali), encamp specifically in the north?
Ramban (in Pasuk 2): Hashem instructed Moshe that Dan should encamp in the north, from where darkness descends upon the world, because he is destined to spread darkness on the world with the Avodah-Zarah which Yerav'am will set up in Dan. 1 Next to him is the tribe of Asher to illuminate the darkness (with oil); 2 And next to him, Naftali, who is a Ba'al-B'rachah. 3 They shall travel last, because whoever worships Avodah-Zarah is last and not first.'
Ramban: See Melachim 1, 12:29. And prior to that, with the image of Michah (See R. Chavel's footnotes).
Ramban: As the Torah writes in ve'Zos ha'Berachah, Devarim 33:24, "veTovel ba'Shemen Raglo".
See ve'Zos ha'Berachah, Devarim 33:23.