I agree with giving da'wah to the soldiers, explaining to them about islaam and maybe even showing them kindness to a certain degree, in hope of attracting them to the truth. BUT i totally disagree with doing it in the mosque. They are, after all kuffaar and the masjid is the sacred and pure house of ALLAH. Don't tell me there was NO other venue availabe to hold their meeting! And even if there really wasn't, i don't think its correct to bring the enemy of ALLAH into the house of ALLAH.