You must drive out the lands inhabitants

One of the main points of this weeks parsha talks about the commandment to conquer the land of Israel. It's interesting to note, that like the acceptence of the Torah, this too was forced upon Bnei Yisrael.
The Gemara in Masechas Sotah (34a) says clearly, as Yehoshua was explaining to the Am that they are going into the Land to conquer it and disposess the inhabitants:
אם אתם עושים כן מוטב ואם לאו באין מים ושוטפין אותיכם
If you do so, then good, if not then the water will come and wash us all away (they were standing in the middle of the Yarden with the water stopped).
This being remincent of Sinai, where the Am was told, accept of this will be your grave (כפה עליהם הר כגיגית). While it is clear why this was needed at Har Sinai, why was it needed here? Doesn't every nation want more land? Don't people want to expand their holdings?
The answer to those questions is yes, but there was another step necessary to fulfil the commandment. That being the destruction of the nations of Canaan. Which was more difficult. Am Yisrael are Rachmanim Bnei Rachmanim, to a fault it seems. It's not only in this day and age that large sectors of the population are willing to give up our rights to our own land, to deal poorly with our own brethern, all in the misguided desire to be merciful to the inhabitants of the land. Yehoshua understood this and made the people promise to conquer their desire to show mercy to the nations of Canaan, since doing so would only be to the detriment of the Am, as is clearly seen from not only ensuing generations, but our own as well.
As the Gemara says, he who is merciful to the merciless, will in the end be merciless to the merciful.
ופוק חזי האי דברים פשוטים המה