A false united front
ופרעה הקריב וישאו בני-ישראל את-עיניהם והנה מצרים נסע אחריהם וייראו מאד וצעקו בני-ישראל אל-יקוק
Pharaoh brought [himself] close. The Jews raised their eyes and behold! The Egyptians are traveling after them! They were very afraid, and the Jewish people cried out to Hashem
The climax of the Exodus was about to begin. The Jews hit a dead end in their escape from Egypt, at the shores of the Reed Sea. They turned around and saw that their dreaded enemy the Egyptians were fast approaching. The Torah describes the Egyptians’ travels in the singular, using the word נוסע instead of the plural נוסעים. Rashi explains that they were בלב אחד כאיש אחד, with one heart, like one person. This shows their complete unity in their plot to annihilate the Jews.
What’s interesting to note is Rashi later uses a similar expression to describe the Jews. When they awaited in anticipation to receive the Torah at the foot of Mount Sinai, it says that they ויחן שם, encamped there. Again, the singular is used to describe the actions of many, instead of saying ויחנו שם. Rashi explains that they were כאיש אחד בלב אחד, like one person, with one heart. Again, this shows their unity in their desire to receive the Torah. However, if we look carefully, we’ll notice that Rashi flipped the expression from what he wrote regarding the Egyptians. Why the change in expression?
One explanation is that the Jews, in their essence, are united. Their souls all really have one unique source. This is why they were first described as כאיש אחד, like one person. All that was lacking until that point was a unified will. Once they united in their desire to receive the Torah, then they were considered having לב אחד, one heart. This is unlike the Egyptians, who at their root, are really divided. In essence, they have nothing to do with each other. It just happened to be, during the Exodus, they formed a united front against the Jews. Their combined desire made it so they had לב אחד, one heart. Once that occurred, only then did they artificially become like איש אחד, one person.
The following analogy hopefully brings out the point. Since a one person has both a left hand and a right hand, it’s possible to say that whatever the left hand wants, the right hand wants. However, you can’t say that with regards to two people. You can only say that if Reuven wants something, and so does Shimon, then they’ll join together to achieve it. Therefore, since the Jews are in their essence one being, that means that each individual is looked at like the left hand or the right hand. They all have the same desire. However, the Egyptians are not like that. Since they are in their essence divided, the only way they can come together is if they have the same desire. Once they have that desire, only then they can be seen as one entity. Good Shabbos
 Based on various sources that I found and collected
 Rashi ad. loc.
 Exodus 14:10
 Ad. loc., partially based on Mechilta and Mechilta D’Rashbi ad. loc. (which says they were כאיש אחד)
 Exodus 19:2
 Ad. loc., partially based on Mechilta ad. loc. (which says they were בלב אחד). See also Mechilta D’Rashbi ad. loc.
 Ma’ayana shel Torah to Exodus 14:10, in the name of the Avnei Nezer
 See Yerushalmi Nedarim 9:4 with Korbon HaEidah, that כלל ישראל are גוף אחד, and Nefesh HaChaim 1:2, 2:17, that the Jewish people share one שורש נשמה
 See also Lev Simcha parshas Beshalach 5743, quoting the Chiddushei HaRim
 Pachad Yitzchak Pesach § 41