Unity Cancels Curses:

The Rebbe of Radzimin zy"a explains that Balak was giving advice to Bilaam. He was telling him that if the nation was "in parts", meaning that they were fractured and lacked unity, then he would be able to "see them", meaning he will be able to curse them. If, however, they would be "all of them", i.e., united and at peace with each other, then he would not be able to curse them.