The phrase 'to assert' is correct and usable in written English.
It is used when you want to state something strongly and confidently. For example: "I assert that our new strategy will lead to success."
Albert would, in the following years, manage to assert the Habsburg hegemony in the Netherlands.
He arrived determined to assert the Justice Department's independence.
Do we need even more research to assert the obvious?
Instead of -ŏ -yo) Koreans sometimes use -ji -yo) to assert the speaker's involvement.
He proceeds to assert the crucial importance of scripts and screenwriters.
Gradually, Mr Karzai has started to assert the role of the centre against the regional commanders.
This is a bid, I think, to assert the show's importance.
It's time to assert the offensively obvious: England really need Gayle's wicket here.
Is Mr Brown prepared to assert the rights of nation states to control capital flows?
Carpenter had done this, a front-page Times story reported, to assert the rights of delegates.
Mr. Bush says he has gone out of his way to assert the contrary.
