In expansionism, states expand their territory through military empire-building or colonialism.
In the classical age of conquest moral justification for territorial expansion at the direct expense of another established polity (who often faced displacement, subjugation, slavery, rape and execution) was often as unapologetic as "because we can" treading on the philosophical grounds of might makes right.
As political conceptions of the nation state evolved, especially in reference to the inherent rights of the governed, more complex justifications arose. State-collapse anarchy, reunification or pan-nationalism are sometimes used to justify and legitimize expansionism when the explicit goal is to reconquer territories that have been lost or to take over ancestral lands.
Lacking a viable historical claim of this nature, would-be expansionists may instead promote ideologies of promised lands (such as manifest destiny or a religious destiny in the form of a Promised Land), perhaps tinged with a self-interested pragmatism that targeted lands will eventually belong to the potential invader anyway.