Now, just because God declared some positive things in regard to Israel, that doesn't mean that he didn't require anything from them. Notice all of the stuff that He commanded them to do... be strong, be courageous, divide the land, obey the law, proclaim the truth, meditate upon the words of the Bible (the book... which at that time was comprised of only 5 or 6 "books" out of our 66), don't be dismayed and prepare for the future. God's promises are always so big that only He has the power to fulfill them, yet we find that He always asks us to do something specific in preparation for the deliverance of or the fulfillment of those same promises. It’s not that our actions merit His favor, but they most certainly are required to demonstrate our sincerity (see all of Hebrews 11 as well as the book of James). Clearly Joshua 1:8 is an if/then statement. If the people would allow God's revelation to dominate their mouth, mind and life... then that would free God up to bless them with prosperity and success. Victory was theirs for the taking.