Joshua 23:5

“ And YHWH your God, he will thrust them out from before you, and drive them from out of your sight. And you will possess their land, as YHWH your God spoke to you.”

They could be sure of success because YHWH their God was with them. It was He Who would thrust them out, and drive them from their sight. Through faith in Him and obedience to the covenant they would find success, and it was only if they had faith and were obedient that they could they expect YHWH to work for them. And the result would be that they would possess the whole land, as YHWH had promised. For these words compare Joshua 1:15; Joshua 3:10; Deuteronomy 6:19; Deuteronomy 9:4.