‘ From the wilderness, and this Lebanon, even to the great river, the river Euphrates, all the land of the Hittites, and to the Great Sea toward the going down of the sun (the west), shall be your border.'
The land was strictly defined. The wilderness is that which they came through on their way from Egypt, the wilderness of Edom, Kadesh and Sin, beyond the Negeb up to the Edom border; Lebanon and the land of the Hittites was the land to the north, roughly up to the Euphrates. ‘The land of the Hittites' was probably northern Syria, called this also in Assyrian inscriptions and the Amarna letters. The Great Sea was the Mediterranean. The fourth border was the Jordan, although some see ‘this Lebanon' as marking the eastern border and referring to the easternmost of the Lebanon ranges, indicated with a wave of the hand even though not in sight.
But ‘all the land of the Hittites' may be intended to be a general term (like Canaanites and Amorites) to indicate Canaan where there were colonies of Hittites. Thus some see it as signifying Canaan, the one nation standing for the many, of those named as inhabitants of the land. (LXX omits the phrase, finding it difficult). Notice the more exact definition of the land to be possessed in Numbers 34:1-15 with the northern border at mount Hor (one of the northern summits of the Lebanon range), Lebo-hamath (or the entering in, or border, of Hamath) and Zedad. Lebo-hamath is now testified to as a city archaeologically.
Under David and Solomon (1 Kings 4:21) the whole area would come under Israel's influence by one means or another (apart from Phoenicia, although that became connected through marriage, and Philistia which was subdued), but they did not cast out their inhabitants, they made them tributary or made treaties with them, and thus when Solomon and finally his sons failed to maintain their position, much of it was soon lost to them. For possession was dependent on obedience to YHWH and it was obedience that was lacking. It is always so with God's gifts. They must be possessed. And if we fail to possess them we lose them.
There is an important lesson here. God did at this stage make the whole land available to them. He promised that it was theirs for the taking. When hey failed to possess it, it was not His promise that failed. What failed was obedience. Thus did they lose what was rightly theirs because given to them by God. We never dream how much we lose through disobedience.