Having played Washington before, knowing the severe lack of petroleum they must operate under, I stayed completely away from mechanized units. Opting instead, for a horde of marines and a few squadrons of F-18s.
I worked on my economy while my army was building. Food, energy, and water were my real money makers getting me about 25 mil a day, with the rest accounting for about 5 mil a day. Eventually I ran completely out of oil, so I spent a few billion to buy new stockpiles of it so I could continue to produce industrial goods. Eventually I did not have the money to buy more petrol, and my industrial goods and petrol were at constant shortages. Didn't slow down my marines though.
After getting about 25 divisions of marines, I massed them on the border with Oregon. They fell within the week.
I then moved them to the border of Idaho, taking a couple of northern nevada towns after they decided to declar war on me. Nevada never bothered me again, given that they are such an isolated region with little to no supply. By the time I arrived at the southwestern border of Idaho with my army, they had declared war on me and I had another 15 divisions of marines at their northwestern border, as well as 5-10 more divisions of reinforcements for my main force. After marching into Idaho's capital, and taking their norther towns they fell, with their eastern city falling under the borders of montana.
A week after taking Idaho, Montana and Utah declared war on me. I moved my army east and took Montana's new city, and fortified my front with Utah from their marauding units. I moved half my army north to a small town in montana, which met with the rest of my army coming from the west. Helena and Boseman fell a day later, and I mover further east finishing off Montana.
At this point I had a small supply of petrol, and I had gathered a good 15 air divisions from my conquests as well as built 12 of my own. I also found that if you sell your pacific fleet, and all the reserve patrol, and troop transport aircraft that you can cash in a cool 70 billion. Which is all quite helpful, but alas not enough.
At this point, North and South Dakota declared war on me, as well as Wyoming. This is where the trouble began. North and South Dakota had much better quality military than any of my previous encounters. And being so far away from supply lines, it was all leading to stalemate.
My first 10 divisions into N. Dakota were cut off from supply, and bombed into the afterlife. I formed a line on wyoming's northern border with about 15 divisions and worked south to about the middle of the region and held my ground. Unfortunately S. Dakota was rather close to an eastern Wyoming town, and as soon as I took it they started pouring into my territory. I stopped their advance, barely. Meanwhile I still had a steady stream of marines pouring in from the homeland.
My second army of marines to venture into N. Dakota again met with tragedy, even though I had 10 helo squadrons and 8 fighter bombers for support. The marines just ran out of supply and didn't move. Then they were routed and died slow deaths. My air force was all but destroyed by N. Dakota's superior air force. Having no more forces on the eastern front, the Dakotas started to make gains into my territory.
Meanwhile, Utah managed to break my line and cut a few divisions off from supplies, which cost me but that area still held. Wyoming began to break through my depleted defenses, and I had fall back to montana to avoid being routed.
And there I held the line, unable to field a force sufficent enough by this time to take any of my abundant enemies. I had boycotts by the WM and everyone else, and all the AI had stockpiled vast amounts of units all with maxed out strength levels. At war with 4 regions, the costs were draining my treasury and I was on the path to destruction.
A few mistakes I made.
I should have traded as much tech as possible with the AI before I went to war with them. Certain recycling techs and other such passive bonus techs would have aided me greatly had I traded for them.
Instead of holding 20 divisions north of Utah, I should have reinforced them with 10-15 more and moved on salt lake city. At the time I had thought that I would get eaten up by a mass of their units(which may have happened anyway). However, had I taken them down, I could have hit Wyoming from the West and dropped them relatively fast. The Dakotas put up much more resistance than I had thought they would.
Given the level of my unemployment(about 8-10%), I should have mined more petrol, even though my fields were rather weak in concentration. I did get handed quite a few mechanized units and planes in taking the land that I did, yet I barely had the petrol to move them from place to place. I had just enough petrol to refuel my planes so they could continue what little air support they provided.