I'm looking for a word or phrase specifically used to refer to the act of placing a flag to claim new territory. I'm specifically referring to claiming land in the name of some sovereign, though a more general term will do.
Due to tylerl's specification, I did a little bit of research, and actually, tylerl, "planting the flag" might just be the expression you're after. I saw it in a blog, :
“Planting the flag” usually means making a claim to something, usually territory or land. Throughout history men have “planted the flag” claiming ownership in the name of the king, queen, country, church, etc. marking the land as their own
In this Space Review edition, it referred to the time when Neil Armstrong reached the moon, as "planting a flag", and says it's "only the beginning".
I reckon it's correct usage.
One might annex a piece of land or territory, which would tend to mean claim in a legalistic way or seize by coup-de-main rather than wage war for it.