ramban on the mitzvah of yishuv ha'arertz

The Ramban in last week’s parsha (33:53) writes that “v’horashtem es ha’aretz v’yishavtem bah” is a mitzvah to conquer and live in Eretz Yisrael.

על דעתי זו מצוות עשה היא, יצווה אותם שישבו בארץ ויירשו אותה כי הוא נתנה להם, ולא ימאסו בנחלת ה'.

He then adds something that I think is a little surprising. Let me explain with an example: taking a lulav on Sukkos is a mitzvas aseh. If I were to ask what a bitul of that aseh would be, I think you would answer (as would I) that the bitul aseh is not taking a lulav. The negation of P is not-P. You wouldn’t immediately say that taking the branch of an orange tree or an apple tree is the bitul aseh, except in the sense that if you do these things you have failed to take the lulav and are guilty of not-P.

So what constitutes a bitul aseh of yishuv ha’aretz? Had you asked me, I would have said it's not settling the land, assuming there is no good reason to not do so. Ramban, however, writes:

. ואלו יעלה על דעתם ללכת ולכבוש ארץ שנער או ארץ אשור וזולתן ולהתיישב שם, יעברו על מצוות ה'.

Why does he talk about planning to conquer some other land and settling there as the bitul of the mitzvah? Even if I don’t plan to conquer some other land and make it my home, the every fact that I didn’t settle in Eretz Yisrael given the opportunity to do so should count as a bitul mitzvah! I’m not sure what to make of what he is saying. It’s almost as if there was an issur aseh involved here, and just remaining passive is not enough to violate the issue – it requires at least making some affirmative plan to do something, i.e. to conquer some other county, in order to reach the threshold of having violated that issur. Notice that he ends the sentence where he speaks about the mitzvah by switching to the negative, "v'lo yimasu b'nachalas Hashem."  Furthermore, it seems that the bitul of the mitzvah can only be through a communal effort (a din in the tzibur) -- the Ramban phrases it in the plural, and logically speaking, conquering a different country is not something that could be undertaken as an individual effort.  Is the chiyiv to conquer and settle Eretz Yisrael a chovas hatzibur, or is each individual responsible to make whatever effort he/she can to live there?  I have more questions than answers here.