To add something more, according to a quick google search sanitariums are most typically associated with the treatment of tuberculosis in the late-nineteenth and early-twentieth century. Tuberculosis can make breathing difficult.

The protagonist never leaves his cell/room, therefore he has no idea what is outside of his walls. He becomes frightened of whatever is outside, causing him to go more insane.

The patients in the sanitariums, were isolated from the outside world for so long, that they: “build their fear of what is out there”.

The lyrics come with the CD version and according to that, the line is “Cannot breathe the open air”. No extra ‘and’ or ‘I’.

He doesn’t day, “And cannot breath the open air.” He actually says, “I cannot breath the open air ”

The sanitarium is dead-focused on locking up those perceived as insane. So, connecting with the previous line, the protagonist dreams of freedom, away from the inhumane experiments and the constant sight of barred windows, keeping them contained.

Seing a locked or inaccessible door in a dream is believed to be a sign that you’re hiding something from yourself.

I see these lines connected to previous one where he wants to be free away from locked doors and windows barred that scarred him.

Also the full moon has always been thought to bring out the crazy in people, that’s actually where the term lunatic derives from.

“Origin

Middle English: from Old French lunatique, from late Latin lunaticus, from Latin luna ‘moon’ (from the belief that changes of the moon caused intermittent insanity)."

Assuming the song is an exposition of the Hemmingway novel (which the band says was the inspiration), there is a pointlessness to the fighting in the story. Robert Jordan doesn’t seem to significantly favor one side over the other and never explains his motivation to join the Spanish army. He fights merely because that’s what he does.

Like SrElite said, it’s about each side believing that they are the good ones, fighting for what is right, against an evil enemy.

The Spanish Civil War is famous for the ‘International Brigades’, foreign national, left-wing intellectuals and Communists, who fought for the Republican side as an ideological struggle against the Fascist falangists. Often, these idealists were severely disillusioned with the heavy authoritarianism (because of Stalinist Soviet Union influences on the republic) that set in as the war worsened, as well as the general misery of war that they experienced. George Orwell is one example and his later attacks on the Communist experience (such as Animal Farm from 1945) shows him clearly disillusioned with Communism as a practical ideology. I think this line draws a reference to this kind of disillusioned idealism through the prism of Hemingway’s book.

The idea with this line is that a proud soldier would do his duty despite the lingering injuries that he has accumulated throughout previous battles. The ‘test’ here is whether they will give in to their injuries or continue to fight.