I want to know your best war or dipolmatic strategy.

Build lots of military units.

An example, or a true strategy? Check out these for actual strategy:

War Academy (http://www.civfanatics.com/civ4/strategy/)

For an example, here's a World War I set up:

Ragnar, Charlemagne, and Isabella were on one continent. Everyone hates Isabella, and she's on the verge of capitulating to Charlie or Ragnar. Me (Pericles), Asoka, Zara Yaqob, and Pacal II are on my continent.

First, Pacal vassalized to Asoka. Next, Ragnar bribed Asoka to go to war with Isabella. Finally, Isabella capitulated to Charlie. He made peace with everyone, and feelings were good. Zara and Asoka were pleased/cautious, so I bribed Zara (who had a military tech lead) to declare war on Asoka and Pacal. Then, I bribed Ragnar to declare war on Charlemagne, whom I bribed to declare war on Zara. So now...

Asoka/Pacal and Charlemagne/Isabella v Zara
Ragnar v Charlemagne/Isabella

I built up an army of Riflemen and Cannons, and declared war on Asoka/Pacal. I captured all of Pacal's cities, made peace with Asoka, then turned my troops around to attack a weakened Zara, who capitulated. Now...
Asoka and Charlemagne/Isabella v Me/Zara
Ragnar v Charlemagne/Isabella

Finally, the other continent made peace. I got Ragnar to switch targets to Asoka, while I made peace with him (he had my entire northern border, a long front), and assaulted Ragnar (backstabbing him), racking up charm with Charlemagne to get some good techs. At the end, I had double the land, a vassal, and a tech lead, while the rest of the AIs were weakened and easy pickings.