What about an "escort" ability? Maybe an early small wonder or just make it inherent to all military units, or just defensive units. It would function like an enhanced "stack move". The "escort" unit can function like a regular unit but has an "escort" option/button (like "Bombardment" is an option). When "escort" is enabled, the player selects another unit which the "escort" then attaches itself to in a semi-permanent "stack move". Wherever the "primary" unit moves, the "escort" moves with it. The "escort" stays attached to its "primary" unit until the player activates it again or its assigned "escorted" unit is destroyed or becomes unavailable. More than one "escort" can be attached to each "primary", and if the movement rate of any "escort" is greater than the combined stack's allowable movement, or if any "escort" has movement left at the end of a turn, each "escort" with excess movement will fortify at the end of the turn, but will resume movement with the stack if its "primary" unit moves on the next or any subsquent turn. Defending during combat is as normal, the stack does not count as an army, so normal unit selection applies (highest defence gets hit first) and stealth units still get their choice of unit to attack. "Escorts" themselves will not attack unless they are activated individually (i.e. removed from "escort" duty).
Example 1
I assign an Aegis cruiser (say, move of 6) to "escort" my carrier (also a 6). From then on, the Aegis will not appear as a "ready" unit but the carrier will, and whenever I move the carrier, the Aegis moves with it. I them move my carrier 6 squares towards another continent, Aegis moves with it. On the next turn, I move the carrier 4 squares and launch airstrikes, after which, I leave the carrier in place. The Aegis then fortifies when I end my turn (having 2 extra movement). On my next turn, I move the carrier 3 more squares. The Aegis moves with it and fortifies at the end of my turn (with 3 extra movement).
Example 2
I assign a destroyer (move 7) to "escort" my transport (move 6). I then send the transport halfway around the world. After each turn on the way there, the destroyer will fortify (having one extra movement), and resume movement on the transport's automatic move. Once they get there, they get attacked by a bomber, which attacks the destroyer (which has higher defence, in this example anyway). After the bomber is done, a submarine attacks and chooses to attack the transport (stealth ability). The transport sinks, and on my next turn, the destroyer is a "ready" unit, since its "primary" has been destroyed.
Example 3
I assign a Mech infantry (move 2) to "escort" a settler (move 1). Then I assign a second Mech infantry (move 2) to the same settler. I then move the settler towards new territory along railroads and then onto an unimproved hill (no roads/railroads). The stack stops, neither Mech infantry fortifies because they have no movement available. On the next turn, I move the settler to a plain. At the end of the turn, both Mech infanrty fortify. On the next turn, I activate one of the Mech infantry (now off "escort" duty) and move it two (or one) squares away, and move the settler to another plain. The remaining Mech infantry fortifies at the end of the turn. On the next turn, I continue moving the first Mech infantry and tell the settler to build a city. Now, the second Mech infantry (left on "escort") has lost its "primary" and becomes a "ready" unit. I can tell it to move, or just fortify in the new town.
With the "escort" ability now described, call what jonathan_95060 described earlier "combined arms". Make it an optional tech (Military Tactics?) off of something during the industrial age (close to Replaceable Parts and infantry, probably). Every land based military unit can now "join" with any other military unit in the way jonathan_95060 described. Each and every unit can be individually activated, and so be removed from the stack.
Make these stacks, similar to WWII sub-divsional Combat Commands (A, B, and R), configurable such that the order of the units within the Combat Command, or CC, determine the order in which they attack. Say, in the Modern age, I put:
one Radar artillery,
two Artillery,
one Modern armor,
four tanks, and
two Mech infantry
in a CC called "Ebo's Fifth", and in that exact order.
If I want the whole stack to attack a city or something, the Radar artillery will bombard first, followed by the two artillery (in order) and then the Modern Armor will attack until destroyed or it retreats, then the tanks, in order, etc. If I capture the city, everything that can still move moves into the city, so I should get something to defend my artillery until the next turn. During my opponent's turn, he attacks my new city, which the Mech infantry defend first since they have the highest defence, followed by the Modern armor and then the tanks, as determined by hitpoints lost and all.
Alternatively, I could select just the Radar Artillery, the Modern armor, two of the tanks, and one of the Mech infantry to attack. If successful, then again, everything that can move does, and if unsuccessful, the remaining units can then attack (if they can) or sit and wait for reinforcements.
Naval battle groups would be tougher, since transports and carriers shouldn't attack. For air units, a tech off of Advanced flight to enable Bomber groups. I'll go into detail on these tomorrow or Sunday, since I have to head out to work now.
remain the CTP system: a free (not linked to a special ability o unit) units assemblage where all units 
Only ground units with a hit point bar can be loaded into armies, like cavaly, tanks, and units that move on two feet. (Mech infantry have two feet, right?) Artillery, workers, settlers, leaders can not be loaded into an army. The little "load" icon does not show up. 
