You should note the cost of the Civics you were running, Vassalage and Police State are High Cost and Pacifism is Zero cost, Pacifism and Vassalage are quite Synergetic in a Specialist Economy, as the negatives of both Civics are canceled out by the positives of the other.
Pacifism has Zero Civic Cost, while Vassalage has high Civics cost
Pacifism has a +1 Gold to Military Support Cost (Military only cost you extra if their outside your cultural boarders)
and Vassalage Gives you Free Unit Cost (Meaning you don't have to pay for your military under that Civic)
You wee running 2 High Civic Cost at the same time (Vassalage and PS) leading to a high Civics Cost which drained your economy, even With a Organized Civ Appearantly.
If you have Problems with Civic Combinations, I suggest playing with a Spiritual Civ and play around with the Civics to get used and to and understand their Benefits and Cost.
What killed me the most was the extra unit upkeep from Pacifism, not the "high" upkeep cost of Police State. A game I started last night has been much more successful so far:
1. Pacifism phase where the beelines are Philosophy and Civil Service, to enable Pacifism and Bureaucracy for wonder-building and GPPs. This extends the effectiveness of a specialist-based economy as well.
2. Military phase initiated either upon attack from an aggressive neighbor, OR... if I've built all the infrastructure I intend to build in 2 or more good production cities, and there's a nearby gimp just waiting to be slammed, AND... my economy's healthy enough to handle the addition of more cities (60%+ slider, no deficit). Here the civics switch to Vassalage and "anything but Pacifism". If I have the Pyramids and have the choice, the government civic is either Representation (if I have a lot of specialists) or Police State (if I don't, or if the need for rapid unit-builds and WW remedies is desperate). Otherwise it's usually Hereditary Rule (happy bonus from garrison).
The goal of the military phase is to get expanded to where I go into the modern era in:
3. Commerce phase. Universal Sufferage; Free Speech; Emancipation; Free Religion; and either Free Market, State Property, or Environmentalism, depending on factors of trade, food situation, or availability of trees. If there is a late-era war, things switch back to military phase until peace breaks out again.
The Problem About War Elephants, the only thing that can truely Counter them before Pikeman are available is the Greek UU (Spearman) that has 5 Str. If your not playing as the Greeks the next Best alternative is you use Formation Promoted Maceman, Which will require 10XP, 8XP for Charismatic Civs and 5XP for Aggressive Civs.
IF you were having a major Problem with War Elephants you could of Pillaged his Ivory or Capture the City that Controlled Ivory to prevent him from building War Elephants.
Usually in an elephant-rush situation, when my economy and civics are properly tuned, I can mass-produce enough regular spearmen such that I can match flood with flood, waves of spearmen fighting waves of elephants, and while I lose about half to two-thirds of my spearmen in those battles (costing me no WW because it's in my culture zone), the survivors promote to Formation, in which case THEIR chance of survival is dramatically improved and I can finally start batting them down at a rapid pace, and become more able to focus on offensive stacks heavy on swords/maces and catapults.
The problem in that particular game was that Pacifism was killing my economy, and the faster I built units, the faster my slider dropped, and the more permanently my tech was stuck in the Bronze Age, while Napoleon continued to research ahead of me. Eventually his stacks switched to Knights, Maces, Crossbows, and Trebuchets while I couldn't even build Macemen yet. I was losing cities without much hope of regaining, so I pulled the plug and took the lesson learned about the dangers of wartime Pacifism.