“And I entreated G-d at that time” (D’varim 3:23). Rashi explains what “that time” refers to — “after I conquered the land of Sichone and Oge,” as well as why it led to Moshe asking G-d to let him enter the Promised Land — “I thought the vow (not to let me enter) may have been undone.” Now that he had conquered land that would become part of the Land of Israel (as evidenced by it already being assigned to some of the Tribes), and he was physically in that land, Moshe thought he might be able to cross over to the other side of the Jordan River as well (see https://rabbidmk.wordpress.com/2010/07/21/parashas-va-eschanan-5770/).
In order to get to the land of Sichone, the nation had circumvented Edom (Bamidbar 21:4, D’varim 2:1-8), and moved north past Mo’av (Bamidbar 21:10-13, D’varim 2:8-9 and 2:18) until they reached Sichone’s border. Moshe then asked for permission to pass through his land, but instead of granting it, Sichone attacked and was defeated, with his land being conquered (Bamidbar 21:21-24, D’varim 2:24-36). The end result (after Oge also attacked and his land conquered as well), was the Nation of Israel no longer living in the wilderness, but in Arvos Moav (Bamidbar 22:1), across the Jordan River from Jericho. It was there that Moshe addressed the nation before his death (D’varim 1:5), renewed the covenant between the nation anf G-d (D’varim 29:9-14), and from where the nation crossed the Jordan. However, their starting point (from where they departed to get there) was Kadesh (Bamidbar 20:22), which is south of the Promised Land on the western border of Edom, and from where, or near where, the spies had been sent (Bamidbar 13:26). Therefore, just as in the first year they would have entered the Promised Land from the south had there been no sin of the spies (D’varim 1:19-21), they should have been able to enter from the south in the 40th year as well, after the generation that had sinned died out. Why did they have to go all the way to Arvos Moav to enter the land from the eastern bank of the Jordan River rather than just going north from where they were, entering from the south, as had been the original plan?
One possibility is based on the need to renew the covenant that had been enacted with the generation that came out of Egypt with the next generation (see Rashi on D’varim 4:45). When the covenant was first made, the nation was at Mt. Sinai, isolated from outsiders, and able to focus on its details. Had the nation been able to enter the Promised Land shortly thereafter, the covenant experience at Sinai was enough to allow them to enter from Kadesh [Barneya]. After 40 years, though, when this same covenant-entering experience was repeated, it couldn’t be done at Kadesh, near others who were hostile to them (e.g. Edom, Amalek and Canaan), as they wouldn’t have the same peace of mind and lack of distractions. Arvos Mo’av, on the other hand, after Sichone and Oge (and the nearby Emori), as well as Midyan, had been defeated and were no longer a threat, and Mo’av had learned its lesson after trying to hire Bilam (so no longer tried to drive them out, for the time being at least), provided a better setting (in some respects) than Sinai (which was in the desert) for the covenant renewal. Although had Edom and Mo’av, or Sichone, let the nation pass through their land to get to Arvos Mo’av it wouldn’t have been as peaceful a setting (as there would have then been hostile neighbors), this will be addressed below, when another possibility is presented.
There are numerous explanations given for why, after the exodus from Egypt, G-d didn’t lead the nation right into the Promised Land by taking “the Way to the Land of the P’lishtim” (Sh’mos 13:17), using a much longer route instead. Rabbi Y’hoshua ben Levi (Sh’mos Rabbah 20:14) compares the situation to a king who had 12 sons, but only 10 pieces of land, so instead of giving the land to his sons and creating tension between them, the king waited until he purchased two more pieces of land, whereby he could give a full tract to each of them. Similarly, G-d said that if he brought the nation straight to the Promised Land, there wouldn’t be a portion for each of the 12 Tribes. Therefore, He kept them in the desert until they were able to conquer the land from Sichone and Oge, so that R’uvein and Gad and half of M’nashe could take it. After that, He brought them into the Promised Land (west of the Jordan River), where the other Tribes would get their full portions too.
There are several things to digest from this Midrash. For one thing, it supports the notion (discussed in the above referenced piece from 5770) that Gad and R’uvein would have (eventually) received that same land even if they hadn’t asked for it. For another, it indicates that the land would have been conquered from Sichone and Oge no matter what; when Moshe asked Sichone for permission to pass through his land, there was no real possibility of Sichone saying “yes” and thereby avoiding being conquered (see Rambam, Hilchos T’shuvah 6:3, that Sichone’s free will was removed). Nevertheless, Moshe is praised for “pursuing peace” (see Bamidbar Rabbah 19:27), although it is unclear whether Moshe knew that Sichone would not, and could not, give permission. The request from Mo’av (Shoftim 11:17) likely falls into a similar category; even if Yiftach used it to show that there was no ill intent towards Mo’av (and, by extension, Amon, see Malbim, Torah Or, on Devarim 23:4), ultimately, Sichone conquered the land from Mo’av so that it could eventually be conquered by the Children of Israel (who weren’t allowed to wage war with Mo’av, see D’varim 2:9). Another aspect that needs to be dealt with is that this Midrash is coming to explain why G-d led us the long way from Egypt, indicating that the delay in the desert was necessary even before the sin of the spies. [This issue is discussed by the commentators on the Midrash.] A related issue is what would have happened had the spies (and nation) not sinned, as the nation would have then entered the Promised Land from the south. If there were only 10 portions available (at the time) for 12 Tribes, how could they enter from the south?
The Land of Canaan is described as being inhabited by seven nations (D’varim 7:1), but Avraham was promised that his descendants would inherit the land of 10 nations (B’reishis 15:19-21). The three additional nations are the lands that were inhabited by Edom (Eisav’s descendants) and Amon and Mo’av (Lot’s descendants), lands that stayed within Avraham’s family, but would eventually (after Moshiach) also be given to the descendants of Avraham, Yitzchok and Yaakov (see Rashi on B’reishis 15:19). The difference between the Land of Israel being comprised of seven nations or 10 nations is reflected in its boundaries as well. There are times when the boundaries are said to extend until the Euphrates River (B’reishis 15:18, Sh’mos 23:21 and D’varim 1:7), and times when its northern boundary is very far south of the Euphrates (Bamidbar 34:7-9), as sometimes the Torah is referring to what the boundaries could have been, and one day will be, and other times refers to what it ended up being because we weren’t ready or able to fulfill our destiny (yet).
When we left Egypt, we were not ready, and therefore not able, to conquer all 10 nations. Before we received the Torah, the only way we could have entered would have been by having Sichone conquer part of the land of Mo’av first (since we couldn’t take it from Mo’av ourselves), and then conquering it from Sichone and assigning it to two (and a half) Tribes. Which meant entering from the east (waiting until Mo’av and Sichone had sinned enough to lose their land, or at least part of it). After we received the Torah, even after we recovered (somewhat) from the sin of the golden calf, we became worthy of conquering all 10 nations (as evidenced by the boundaries given in Sh’mos and the beginning of D’varim), and we could have/would have entered from the south, and conquered the lands that had, until then, been given to Edom, Amon and Mo’av, thereby having enough for all 12 Tribes. After the sin of the spies, though, we were no longer able to conquer those three lands, so couldn’t enter from the south, as there would have only been enough for 10 of the 12 Tribes on the western side of the Jordan. We couldn’t conquer land from Amon/Mo’av, and Sichone wouldn’t have attacked us (as he only attacked, and was given prominence by the nations on the eastern side of the Jordan, in order to prevent us from entering the Promised Land, see Rashi on Bamidbar 21:23, and in this scenario we would have already been in the land; it is also unlikely that Sichone would have taken land from Mo’av if there was no need for them to be the ones to block us from entering the land). [If we had attacked Sichone rather conquering his land after he attacked us, Amon/Mo’av would understandably claim that we would have attacked them too had Sichone not taken their land first; Yiftach was able to take the high road because Moshe had asked permission to peacefully cross through but was denied, and because we didn’t attack Sichone and Oge, but they attacked us.] Therefore, after the sin of the spies, we had to enter from the east, conquering the land Sichone had taken from Mo’av, so that there would be enough for all 12 Tribes.
From this perspective, we can understand not only why we had to travel around Edom and then continue north to enter the Promised Land from the east rather than just entering from the south, but why Moshe thought that perhaps he would be able to cross the Jordan River. After all, he had helped conquer, and was already within, the land of two of the 12 Tribes. Why shouldn’t he be able to see the rest of the land too?