Oh, grant us help against the foe, for vain is the salvation of man! With God we shall do valiantly; it is he who will tread down our foes.
David's hope for victory against Israel's enemies was not secured by his own military prowess or the skills of Israel's armies. He knew that to trust in mankind for deliverance was a grave error. He put his trust in God and wanted the nation to know God as their only hope of salvation. It was foolish to only trust in their own strength.
But notice that David did not expect to call off his soldiers and sit back and let God miraculously kill off the foes. No, the confidence was: "with God we shall do valiantly." God would lead the men into victory, but they still had to fight the battles. They still had to do something. They had to go forward into the fray. The balance of faith and action still required that Israel have faith while taking action. And that is where God will be known.
Thankfully, I don't need to fight with sword and shield (at least not literally) to maintain my life. But I do need to take action. I don't just "let go and let God". Faith requires steps on my part in trust of God. I still must make a living, fulfill God's purposes in obedience to Him, share a living faith with others, and do things that require work on my part. But I do it in trust of God and find Him to faithfully fight alongside me, for me, providing and often doing great things with me so that I can do valiantly what I never thought I would do!