Military units vary in thier ranks structure as much as anything else. For instance a Corporal in the US Marines usually commands a fireteam, but in the British Royal Marines a Corporal commands a squad (which they call a section). This makes it impossible to properly compare different ranks between different militaries.
A Captain in the Marines is not the same as a Captain in the Navy. This makes tradition interesting, since there is only one Captain on a ship what do you call a Marine Captain who commands a company of troops? Major, the next rank up (c’mon ya can’t demote the fella, he gets a brevet!).
Below is an attempt compare the rank structures of different militaries with the US. It should be noted that some militaries have different classes that have no comparison with the US. For instance, in India, there is a class of ‘junior’ commisioned officers, and the Cubans also have a sub class of officers. This list is by no means perfect! (although I would appreciate comments on how to make it so!) This list is designed to give you some idea of how things work. You should consider doing some in depth research on the country in question if you are going to write about it in any kind of detail.
Until recently, the Chinese did not recognize different ranks because of their communist view. Soldiers were recognized by their position, like Company Commander, Squad Leader, ect. The Soviet Army did this once also, but quickly abandoned the method because it lowered morale and just didn’t work. The reason this was done was to stay in line with the communist theory that everyone is equal but responsibilities are different. Assigning rank was a very definate way of saying who is better than who.
The Soviet Military has many similar ranks but there is a great deal of distinction between an Engineer Captain and a combat arms Captain and the two are not interchangeable like in many other militaries.
In a combat environment a person may be given a ‘brevet’ rank, like brevet Captain. Basically this is a temporary promotion (or demotion) and lasts only for the duration of the assignment or duty.
The Cuban Military is called Cuba’s Revolutionary Armed Forces (FAR). World War 2 ranks are listed as WW2. In the WW2 listings G- stands for Germany (or as the Natives call it Deutschland). Kreigsmarines were the Deutsch Navy, Wehrmacht was the regular army and Luftwaffe was the airforce. The SS was of course the elite. The Geheimstatspolizei (Gestapo) and Sicherheitsdienst (SD) ranks are the same as, or similar to, the Waffen SS.
In the Japanese Military an additional rank exists between E-2 and E-3 called Joto hei.
WW2 Red Army Air Force: Starshiy Serzhant (Senior Sergeant); Starshina also roughly corresponds to this rank.
WW2 Red Army Air Force: Mladshiy Leytenant (Junior Lieutenant) also corresponds with Midshipman.
In the US Armed Forces, the ranks of General of the Air Force, General of the Army and Fleet Admirals are refered to by the US as Five Star ranks. These ranks were created during WW2 to give parity with the British Commonwealth rank of Field Marshal, Marshal of the RAF, and Admiral of the Fleet and are very rarely rewarded. General Schwartzkopf of the Gulf war was considered for the rank of Five Star General, but not given the rank. It is said that George Washinton was given a unique Six Star rank.
The rank of Specialist (notably in the US Army starting at Corporal) is a non-command designation. In effect a specialist is nothing more than a more higher paid, respected private first class who is not really trusted with a command billet. At one time the US Marine Corps used the specialist rank structure but it was phased out.