“There shall not any man be able to stand before you all the days of your life. As I was with Moses, so will I be with you, I will not fail you, nor forsake you.”
God's promise to Joshua was that he would triumph wherever he went, not necessarily always immediately, but always in the end. Furthermore He would also be with him as He had been with Moses, guiding, advising and strengthening, protecting against all comers. He would not fail him. He would not desert him. He would always be able to be sure of YHWH's backing.
“There shall not any man be able to stand before you.” Compare for this Deuteronomy 7:24. ‘All the days of your life.' Compare Deuteronomy 4:9; Deuteronomy 6:2. These promises are always available to those who look to Him and obey Him when they are engaged in serving Him truly.