What is "u'va'Midbar asher Ra'isa ... " referring to?
Rashi: It is a continuation of the previous Pasuk - 'Hashem will fight for you like all that He did for you in Egypt ... and subsequently, in the desert ... '.
What did Yisrael actually see?
Targum Yonasan: 'And in the desert you saw highly venomous snakes and how the Cloud containing the Shechinah killed them'. 1
Na'ar Yonasan. See also Peirush Yonasan.
What is Moshe referring to when he states "Ka'asher Yisa Ish es B'no ..."?
Rashi: It refers to Hashem's protection of Yisrael. Refer to Sh'mos, 14:20:1:1*.