If you’re willing to be disagreeable with someone else’s opinion, then your posture of assertiveness will work in a conflict situation.
If you’re unwilling to be disagreeable in someone else’s opinion, then your posture of assertiveness won’t work in a conflict situation.
As a matter of fact, it will appear to the other party as if you’re faking it.
Being willing to take a stand, to commit to an action, and to take a chance that you might lose out on being liked by the people you seek to change, these are the behaviors you will need to be assertive in a conflict.
Otherwise, they are the very same traits that, if not applied, will cause you to chose a different approach, which may lead to a different—and undesirable—outcome for you.