What are the precise requirements of the "Model of a Modern Major-General" achievement in Civilisation 5?

Solution 1:

You need to build all the basic units, plus all the civilization Unique Units from the original release nations (no DLC). I'm sure the UUs are required as I popped mine on a Naresuan's Elephant.

It was a surprise to me, but the achievement logging has apparently been re-enabled at some point so you can check on your progress. Here's how (Hat tip to this CivFanatics forum post):

1. Navigate to My Documents/My Games/Sid Meier's Civilization 5 (my path for Win 7)
2. Open config.ini and change the LoggingEnabled line from 0 to 1
3. Run the game and it will create a text file called "achievements_debug.log" in your Logs directory under that Civ 5 folder

Interesting note, the Spaceship parts are also tracked in this log, but I'm not sure if they're required or not for the achievement.

Solution 2:

Building means constructing at a city or purchasing at a city, gifts and upgrades don't count. There was once a bug preventing Lancer from being recognized so people were not getting the achievement, but that has been fixed.