An Anti-Expansion Movement is an inevitability in almost every intelligent civilization or species in the universe and beyond, assuming that the civilization or species in question does not operate in a hive-mind, or is completely loyal to itself. Be it from as small as a species first steps outside of their continents borders, to conquering entire galaxies, there always seems to be a section of its populace who is staunchly against the idea.
The reasons given by an Anti-Expansion Movement can vary depending on the movement in question, though they typically seem to fall under two categories.
One: Is in a sort of practical sense. Either framing their arguments with the expense or time being not available, or too costly to invest in. Other variations of this can be that their culture will become too diluted and ultimately forgotten. Or, that something stronger may exist in where they wish to expand, which may end up destroying them.
Two: Is a moral standing. With them claiming that the civilization or species in question does not have the right to encroach into other territories. These arguments are most typically used when a civilization spreads out to other worlds that contain a native life form(s) of some sort, though this is not absolute.
One can say that the practical side of the argument of anti-expansion is a fair stance to bring up as if it's not in the civilization's or specie's best interests, or is simply impossible at that time, then the respective expansion shouldn't be preformed either entirely, or not at the extent being suggested.
However when it comes to the moral side of the argument, then that is where the waters of debate can be muddied, as morality in its truest form seems to be subjective. Though that in of itself is debated heavily by civilizations as well.
Who is anyone to say that a species conquering entire galaxies is wrong or not?
Though, almost every intelligent species in existence seems to expand regardless of this debate, leaving us to come to the conclusion that expansion is just a fact of intelligent life, that only the civilization in question's destruction can stop.