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, :
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.