[Boromir88]

This seems to be reflected in the part of LotR where the various vassal lords are sending such small pittances of soldiery to Minas Tirith to help.~lmp

Or it could just be people fled from going to Minas Tirith. There is the part in Minas Tirith when the Lords are coming in with there armies and the statement 'always too few than what was expected.' This could mean maybe what you said, or perhaps people just fled not wanting to fight. That was also I think a possibility, considering soldiers did flee before reaching the Black Gate (whom Aragorn kindly let go); and also there was a problem of soldiers fleeing from their posts during the Siege of Minas Tirith:

'Why? Why do the fools fly?' said Denethor. 'Better to burn sooner than late, for burn we must...'~The Siege of Gondor

Or it could just be the people expected more soldiers, when really there weren't any more.

Who is it that actually controls the military?~Kuru

Good question...it is the Lords that march in their army, and perhaps they control their own soldiers, but I think all still remained under the command of the Steward (or King). For Denethor could very well be out there leading the soldiers; he just chose not to and it was Gandalf that took over:

So it was that Gandalf took command of the last defense of the City of Gondor...~ibid

Then later Gandalf passes on this duty to Imrahil:

'You will need every man and mor,' said Gandalf. 'Make all haste. I will come when I can. But I have an errand to the Lord Denethor that will not wait. Take command in the Lord's absense!'~The Pyre of Denethor

It probably should have been Denethor:

'The first circle of the City is burning, lord,' they said. 'What are your commands? You are still the Lord and Steward. Not all will follow Mithrandir. Men are flying from the walls and leaving them unmanned.'~The Siege of Gondor

It seems like the King/Steward is the one that leads the army. When Boromir was still alive and well in Gondor he seemed to be the leader of the troops. One of his titles was the Captain-General which would kind of be like the Commander-in-Chief of the modern day. The person that led the armies into war.

Maybe it's something that depends upon the situation. A large and epic scale war such as the one's the Gondorians were facing here, it's much easier and time efficient to have one leader in charge making the decisions...and not several lords doing their own things with their armies. It's kind of like Shakespeare's Macbeth. Where there are the several nobles that have their armies and they are all brought under command of the Earl (for the life of me I can't remember the Earl's name in the story) when the armies are summoned together.