Guys, don't make a mountain out of a molehill; it was just a mistake due to my inexperience with online play...
My opponent cast Armageddon. I had the Sphere of Vulnerability. And I thought that this would remove all immunities, including those of his dragons. And even if HE were to cast Armageddon, that's why I attacked.
In other words, I was counting on my opponent simply eliminating all his dragons when he cast Armageddon. But then, I also had the bright idea to attack with my main forces, and that's how I lost the game.