I've heard mostly anecdotal evidence that Americans would frequently shoot at their allies in the second world war. For example, the saying: "When the Germans shoot, The British duck. When the British shoot, the Germans duck When the Americans shoot, everybody ducks."
Is this reputation justified and were Americans more likely to shoot at friendlies than the other allies?
I can maybe offer some explanation as to why US friendly fire happened and why they gained a reputation for it.
The Allied invasions of Sicily and Italy saw the first time that American and British troops had fought aggressively in the European theatre of World War Two. For troops who had gained all of their experience fighting in the African theatre, warfare had now become vastly different, going from fighting in sparsely populated deserts to fighting from village to village. The battle-lines were a lot less rigid and therefore open to more confusion.
This was paired with the new-found utilisation of close air-support to wreak havoc on enemy infantry. Close-air support was little used by American forces in Africa so its introduction during the Sicilian invasion had some teething problems, one of which being that the Army could not communicate with the radio frequency used by the Airforce at the start of the campaign. This all caused a level of confusion that led to the deaths of many US servicemen during the invasion of southern Europe. General Omar Bradley's own armoured column was fired on during the invasion of Sicily, forcing them to retaliate by shooting down one of the attacking planes.
As the relatively sparse settlements of Sicily and southern Italy saw bad friendly-fire because of battle-line confusion, the problem was only amplified in the close-packed villages and towns encountered in the invasion of France in June 1944. In the breakout from the Normandy beachheads, Operation Cobra, it is estimated that over 700 US servicemen were accidentally killed by allied troops.
Another separate problem, the US introduction of camouflage uniforms for some units of the Second Armoured division during the bocage fighting in France in 1944. This led to several tragic friendly fire incidents, with some soldiers mistaking their own troops for similarly-uniformed SS troops.
The quote you present is not about friendly fire but about the increadible effectiveness of the US artillery system.
The US based their artillery system on the French post-ww1 system, where a battery would deploy and immediately start to calculate all possible fire missions to be prepared once their support was needed (be it harassing fire, barrage fire, defensive support fire or counter-battery fire).
The US decreased industrial tolerance to the extent that one gun behaved like the next, did land surveys or paid for governments to do their own land surveys in western Europe and pre-calculated many if not most of the possible fire missions. With detailed maps, decentralised forward observing (most US NCOs and officers recieved rudimental training in forward observing for artillery) and portable radios for every platoon (walkie talkies), the time from a frantic call for fire support could be reduced from 15-30 minutes to as low as 30 seconds (in ideal conditions), although 2-5 minutes were more common.
The US artillery could also put effective barrage fire down immediately - normally, the attacking force would be warned by a single or two shells dropping that artillery was zeroing in on them (and that a forward observer was somewhere within direct line of sight) and could withdraw, press the attack, take cover or try to take out the forward observer.
The US artillery could do effective fire immediately, and even tiem on target missions, where the same guin would fire several Shells which would land at the same time.
US artillery observers and fire support staffs could also command many batteries - from other divisions and corps and army artillery, which meant that they could put down unpredecented defensive fire on a single spot at any given time.
Add the fact that the US had one of the best supply systems during the war and could bring more guns to the battle and more shells to their guns, and you have a devastating combination.
This was devastating to the Germans, especially to their knee-jerk counter-attack doctrine where they would counter-attack an enemy that had just driven them away. Normally this would be very effective, as the enemy would not have support and heavy weapons in place, would be low on ammunition and exhausted from the attack, and above all probably not have forward observers in place and the new positions registered in their fire support plan. The US artillery was often able to devastate these counter-attacks.
The US artillery would put defensive fire VERY close to its own troops with as little warning to them as to the enemy - thus when the US artillery fired, EVERYONE ducked.
