1)
Why did he say "Ase Chazak la'Milchamah"?
1.
Malbim: If you want to come with your strength, make strategies and strengthen yourself with many soldiers - but it will not help, for Hashem will make you fall in front of the enemy.
2)
Why did he say "Lazor u'Lehachshil"?
1.
'Rashi': He can help you without the soldiers of Efrayim, and make you stumble if you take them with you.
2.
Malbim: Yisrael does not succeed via nature, rather, via Hashem. He helps those who trust in Him, and makes stumble those who rely on human strength.
