What are the connotations of "Ki Somar bi'Levavcha"?
Rashi, Targum Onkelos and Targum Yonasan: It means 'Perhaps you will say "These nations ... " '. 1
Seforno: It means if (or when) you say ... , don't say it as an expression of fear, but in a way that suggests that it would be impossible to defeat them without Hashem's assistance. 2
Because it one were to translate it as "If ... ', (or using any other of the other meanings of "Ki" - See Sifsei Chachamim), the continuation of the Pasuk "Lo Siyra meihem" would make no sense.
Seforno: Bearing in mind what Hashem did to Par'oh and the Egyptians who were superior to you in number and in strength - yet He delivered them into your hands and you destroyed their name.