Looks to me like you're playing at Regent level.
My advice is to immediately obtain some mutual defense pacts ('mdps'). Other civs tend to be much more open to a mdp than they are to an out-and-out alliance. Except for the civ that's already at war with your enemy. They will almost assuredly go for an alliance with you.
FIRST: Get some mdp's from
non-warring civs. This is crucial for you because you believe your enemy is stronger than you. Sounds like you need some help in your war. Get the other civs, especially those on the same continent, hooked up with mdps and rights of passage. Get your mdps
even if you have to pay for them. Get your mdps even if that civ already has an mdp with your enemy. It's ok.
SECOND: DON'T DECLARE WAR OR TAKE WAR EQUIVALENT ACTION AGAINST YOUR ENEMY, such as pillaging. Instead, move all the units you want to use for your assault against your enemy into the enemy's territory. Your enemy will demand that you move them out or declare war. DON'T DECLARE WAR AND DON'T MOVE YOUR UNITS OUT OF ENEMY TERRITORY. If you declare war, then your mdps are no good, and if there is an mdp existing between your friendly civ as well as your enemy civ, then your friendly civ goes to war
against you.
Just choose the ignore option. Your enemy will then declare war against you. Once that happens, your mdps will kick in on the next turn, and you will have help in your war against your enemy. The key to warfare is getting the other guy to declare the war. Then you are the "hero," and the other civ's declarations of war work in your favor, rather than against you.
Also, I wouldn't pillage the enemy's improvements necessarily. If you've got some strong allies coming to your aide, then wait for your allies to show up and fight. Your allies will typically carry the brunt of the warfare if you have the luxury of waiting on them. Your allies will fight the enemy in the field while you concentrate on defeating the enemy's weakened cities. Once you take the enemy's cities, you'll be glad the improvements are still there.
THIRD: On the same turn your enemy declares war with you, go ahead and approach the other civ that's already at war with your enemy about an alliance against the enemy. Since that civ is already at war with your enemy, they will undoubtedly go for an alliance. But note further: Be sure to see if your prospective alliance civ will pay you gold per turn for the alliance. Usually you can make some coin for 20 turns if you come in on the side of a warring civ. So get paid for the war.
FOURTH: Once war is declared, make sure to go back with all the nations involved on your side, and get them to agree to an embargo against your enemy. An embargo will help you remember when your 20 turn commitment is over, plus it helps keep your allies in lock-step with you.
FIFTH: Don't forget that your mdp/alliance commitments last for 20 turns. Even if your allies make peace with the enemy in violation of the 20 turn commitment, DON'T MAKE PEACE EARLY. Keep your 20 turn commitment. Keep your reputation intact.
But get out of you mdps and your alliance immediately after 20 turns so you don't get sucked into unintended wars with other civs. You will get sucked into other confrontations if you forget to dump your mdps and alliances as soon as possible. Get out of them even if you continue warring with your enemy. Unless, of course, you consciously want to continue the commitment, which, in my experience, is doubtful after 20 turns. I find that after 20 turns the enemy tends to be highly damged and I can finish him off with no further help. Then I want my allies out of my territory before they come after
me!
Hope this is of some help.
Comments / criticizisms / flames requested and encouraged.