http://revach.net/article.php?id=2431

Section: Parshas HaShavua   Category: Quick Vort
Parshas Chukas: Rav Yehonoson Eibushitz Amalek Tries The Tricks of Shimon and Levi... With Their Own Twist
The Torah tells us that the when the Kna’anim heard that Aharon died and the Ananei HaKavod went away, they attacked Bnei Yisrael.  Rashi tells us that this was really Amalek, but they spoke the language of Kna’an although they were dressed like Amalek. Why would Amalek attack again after having been thoroughly trounced the first time?  Why did the disappearance of the Ananei HaKavod give them hope?  Why did they speak Kna’ani?

Rav Yehonoson Eibushitz answers that Bnei Yisrael did not perform the Mitzva of Bris Mila in the Midbar because the Ananei HaKavod prevented the Ruach Tzefonis, the northern wind from blowing.  This wind is necessary for the healing process, and without it having a Bris Mila would be too dangerous.  Now that the Ananei HaKavod went away, Amalek was sure Bnei Yisrael performed Bris Mila at the first possible opportunity.  The figured that in Bnei Yisrael’s weakened state after the Bris Mila, they can defeat them just like Shimon and Levi did to Shchem.

The reason for the attack was that Amalek was a grandson of Eisav, and they wanted to avenge Yaakov’s stealing of the Brachos.  To receive the Brachos from Yitzchok, Yaakov wore Eisav’s clothing but spoke in Yaakov’s voice.  Therefore, to raise the level of passion for the fight they chose to carry the banner that would remind them of this episode, and they as well wore one nation's clothing and spoke another nation's language.