Mmm, the best time to start attacking everyone is within the first three to four months. I would spend the first month or two training, and then try to attack one of the neighboring kingdoms. You can then split up your men (if you recruited enough guys by conquering another kingdom) and attack two or three castles at once. I used sort of an "Iron Curtain" strategy, where I'd take over a whole section of the region and block off enemies by putting my best men on the front lines. It was kind of hard to do from the middle of the continent, though. When possible, I equiped one of my best guys (namely the story's signature warlord) with an item called Gaurdian Charm (it maxes out your attack and defense stats if your the last/only Warrior left as well as increases your range by one square)and sent him in alone to take out the enemy's army in a single kingdom. It may sound like cheating, but with the right Warrior, that strategy was extemely effective.
I think I only got one real game over (with one of the three guys who introduces you to the world of the game). However, I found myself having to restart a couple different missions because Nobunaga's and Kenshin's armies got too overpowered by the latter half of my campaign (I wasted too much time trying to see what things I could do as far as developing my nations went).