Two Apache gun ships sitting at "stand of distance", can change the course of battles even to this day! Imagine them in any era before now!
Hell, any of today's armored vehicles placed out of era would have been almost impossible to take down.
You are forgetting something very important.
Those things all consume very large amounts of fuel.
A tank consumers huge amounts of fuel.
.6 mpg for a modern main battle tank and depending on the terrain possibly even worse.
Where are you going to get fuel? Are you going to bring your own refinery and start drilling?
Because even if you get the oil you won't be able to do much with it without a refinery.
Are you aware how many maintenance man hours are involved in a helicopter?!
An Apache under a perfect estimate has many hours of maintenance per hour of flight, and that is with people who know what they are doing while working on it.
Real world numbers can often be 10-15+ maintenance man hours per hour of flight.
You would have a lot of hangar queens very quickly.
Here is a story about using them in southern Afghanistan, where the Apaches supposedly took 32 maintenance man hours per hour of flight!!:
http://www.airspacemag.com/military-aviation/Apache.html
Our information comes from Ed Macy's book, page 45: “[The Apache] needed thirty-two man hours of maintenance on the ground for every hour flown
Helicopters also burn through a lot of fuel themselves.
Military machines use massive amounts of fuel, and some take very intensive maintenance.
A combustion engine pulling a massive 60-70 ton armored vehicle with enough torque to go over obstacles on tracks is thirsty.
There is also few cleared roads back in time, and those that did exist were a lot narrower before automobiles.
For example huge portions of the US which are now sparse were also covered with dense forests and swamps back then. Very thick forests which would make armored vehicles difficult to deploy.
Most modern military vehicles come with a lot more people than those operating them in combat. It takes teams of people who maintain them for a living to keep them up and running between fights.