Aragon
At the beginning of the Spanish Civil War a battlefront was established at Aragón. The first major campaign took place at Aragón in June 1937. The purpose of the Republican offensive was to draw the Nationalist Army from Bilbao. The first battle took place at Huesca where the Republicans suffered 1,000 casualties. The campaign was a failure and General Francisco Franco was able to enter Bilbao on 19th June.
Primary Sources
(1) Jack Freeman, member of the Abraham Lincoln Battalion, letter from Aragón (22nd October, 1937)
We moved into the trenches one morning before light and, as soon as dawn came, the crap began to fly. Then started my education. Some of the old-timers explained the various sounds to me. At first anytime anything whizzed, whistled, or buzzed, I would duck. Then I found out that any bullet which passes anywhere near you will whistle. Ricochets, that is, bullets which have already hit the ground or a rock or something and bounce off in a different direction, buzz when they go by. When bullets come very close they sound more like a whine than a whistle.
But the most important thing of all about these bullet sounds is never to worry about any bullet you hear. Bullets travel much faster than sound, strange as that may seem, and the bullet is way past you by the time you hear it. As it's put out here, "You'll never hear the slug that gets you."
Of course, it's pretty hard to control your instinctive tendency to duck when you hear a loud noise, but the only time it really pays to duck is when you hear a burst of machine gun fire and hear them come over you. You can't, of course, duck the first few if they're coming at you, but you can get out of the way of the rest of the burst.
Well, the first morning I'm keeping low in the trench and not too much interested in the intricacies of military education, when these trench mortars start coming over. They whistle for a long time before they hit and that just increases the agony, waiting for them to land. When these things start coming the battle commander shouts "Everybody down in the trench." So I stick my nose six inches below the level of my heels and then the commander finishes his sentence, "That doesn't go for the observational staff. Locate that gun."
So I found out what observing under fire meant. Poor me has got to spend my time sticking my nose through peep holes when it's much more comfortable two feet below, and my head and shoulders over the parapet half the night, and when the big bastards come over instead of dropping we've got to watch. It was pretty tough the first morning but I soon got used to it.
You see, after a while you get the feeling that what's going to happen to you, if anything, will happen pretty much in spite of anything you do. That doesn't mean we become dauntless heroes and walk out of our way to take risks because we like to watch the patterns the bullets kick up in the dust, but it does mean that we don't become nervous wrecks bobbing up and down every time a mosquito buzzes around your left ear. It's the only kind of defense mechanism you can adopt.
Shortly after noon that first day we went over the top. For about three quarters of an hour after the beginning of the attack I didn't think I'd get a chance to climb over that hump. I was stationed next to the commander in a pretty exposed observation post keeping wise to how our boys were going, so that the attack could be properly directed. The commander, you understand, does not move up until the troops have taken up a position, even a temporary one, in advance of the original lines. But if you think that's safe, you're cock-eyed. He's got to keep calm and see everything that's going on when every instinct is pulling him down to a covered position.
Communication with the men out front is maintained by runners. Pretty soon we ran out of runners, so I got my chance. But the company I had been sent out to contact had had some tough going and was pretty well scattered and difficult to find. I went out, couldn't find the company commander nor anyone else who knew where he was. So I was in a fix. I didn't want to return until I had contacted them and I couldn't find them. I roamed around that god-damned no-man's land, sometimes running, sometimes crawling, sometimes snake-bellying, and holy cow, was that a time. I didn't of course know where in hell my men were and one time I crawled up to within fifty meters of the fascist lines before a sniper reminded me where I was.