Dragons

Dragons are incredible allies to win a war. Let's look more closely at each of them.

 There are four type of dragons, each one negatively affects the enemy kingdom of his own form:

Sapphire

25% lower magic effectiveness
25% lower thievery effectiveness

Emerald

20% Higher military losses in combat
15% lower attack gains

Gold

Reduces building efficiency by 25%

Ruby

Reduces military effectiveness by 8%

Furthermore all dragons result in 10% lower income and the loss of 20% of new draftees.

Let´s think about the strength of each dragon:

Sapphire
The sapphire is very disturbing and I personally think it should be used on almost every war. On the first ticks of the war is probably the moment when more concentrate the use of magic and thievery operation because normally a lot of players are online, with 100% mana and stealth and starting to put their T/M strategy on action. So start the war with a Sapphire normally is a great deal. Also realize that even if the enemy KD have a strategy to kill the dragon very quickly the sapphire will do his job on the short duration it has, open the enemy for our T/Ms on this first rush of spells and operations. Of course a sapphire gains even more importance if we are at war with a kingdom with strong T/Ms.

Emerald
The Emerald Dragon acts in two fronts. It diminushs the land gained by the enemy and increase their military losses. The fact of the enemy take less land will difficult our provinces to become overpopulated. The higher military loss on combate will also decrease the military force of the enemy if it runs for a long time. I think this Dragon should only be used against KDs that have their attackers based on orcs because orcs have more gains on attack and can be ambushed easly so we can kill even more of their elites. It is very bad to be used against undeads or clerics.

Gold 
If you have any doubt of what Dragon to send, go for a gold one that there is no error. The Gold Dragon affects all types of provinces and acts specially on the economy of the Kingdom. A Gold Dragon gain in importance when fighting a Kingdom with good amount of dwarves, because the -25% BE will be apllied over a high BE (remembering that it is multiplied). Also the Gold Dragon can be used to starve the Kingdom since everyone will be producing -25% food (another weakness for the dwarves)

Ruby
The Ruby Dragon was for a long time considered the best dragon to send on the enemy. It allows better chains and attacks on general with the reduction of enemies defense and also offense. Realize that is implicated in its bonus that each attack the enemy does he also will have an increase of 8% of military losses (since will need to send 8% more troops) and a reduction of 8% loss on our own attacks (since 8% less troops). It's a dragon that will surely have its usefulness in any situation but I personaly think that it should be used on the specific situation when we need to break an unbreakable or want to ensure our unbreakable will not be hitted by the enemy. Possible he can be used when you want to make a quickly war allowing your Kingdom to do more chains in a shorter time.

Killing a Dragon 
I normally prefer to have a strategy to kill the dragon very quickly. If we have a good strategy for that we can take it on the first tick without large losses to our Kingdom. The only situation it may change is if we realize the enemy has a capacity of send more Dragons than we are able to kill, but if this is the situation I think is a mistake fight these guys.