How is ISS commander chosen?

Question

I've been looking at the ISS's List of International Space Station expeditions, and I'm curious about how the commanders are chosen. I can't really find anything that stands out. Sometimes the commander is military (or former military) and sometimes they're civilian, so military rank doesn't seem to be what decides it. It also doesn't even seem to be that the most experienced astronaut/cosmonaut becomes commander.

For instance, for Expedition 40 Steven R. Swanson was the commander, but Aleksandr Skvortsov went up with him for Expedition 39. Skvortsov had military experience as a leader, he logged more time in space than Swanson, he was a cosmonaut before Swanson became an astronaut, and Skvortsov even had experience as a commander on the ISS during Expedition 24, while Expedition 40 was Swanson's first time aboard.

I can't really find anything that says why a particular person is chosen to be the commander of an Expedition. How are they chosen, and is there any difficulty among countries/agencies when deciding who will be commander?

Answer 1

Partial answer:

While I can not say anything on the individual qualifications for the cosmonauts and astronauts that became commanders, I do have a clue about the nationality pattern.

It starts off in a pretty simple pattern, with alternating Russian and American commanders for the first 19 expeditions. (With a few exceptions, the schedule was a bit messy in the early construction phase)

Then, on expedition 20 in 2009, a Russian commander followed the previous Russian commander in order to compensate for the ESA commander, Frank De Winne. An American commander followed for compensation. This was the start of the next pattern, with commanders alternating in pairs (Russian, Russian, American, American).

That continued on to expedition 35, when the Canadian cosmonaut Chris Hadfield became commander. Also, the JAXA cosmonaut Koichi Wakata became the commander on expedition 39, and this two events must be compensated. That was done by having three Russian commanders in a row on 36, 37 and 38, and the pattern is now back to alternating American and Russian commanders. The American compensation is currently ongoing, with astronaut Scott Kelly's double reign as commander. (Also connected with the year in space experiment).

The last compensating event scheduled is an American commander on both expedition 50 and 51.