Jamieson, Fausset and Brown on Judges 1:2

2. the Lord said, Judah shall go up--The predicted pre-eminence (Ge 49:8) was thus conferred upon Judah by divine direction, and its appointment to take the lead in the ensuing hostilities was of great importance, as the measure of success by which its arms were crowned, would animate the other tribes to make similar attempts against the Canaanites within their respective territories.

I have delivered the land into his hand--not the whole country, but the district assigned for his inheritance.