For most of the war, American armored doctrine (at least nominally) was focused on infantry support, leaving the job of countering enemy armor to fast, hard-hitting tank destroyers; how effective was this in practice? Would it have been a better choice, in retrospect, to have pursued a more "modern" armored doctrine, one that focused more on tank-to-tank combat?
uh.
This appears to be a popularly repeated myth. The US War Department Armored Force Field Manual at no point says that US armor was inadequate enough that dedicated tank destroyers had to be brought in.
(http://www.ibiblio.org/hyperwar/USA/ref/FM/PDFs/FM17-10.PDF)
Fact of the matter was that every unit the US could field- any army for that matter- had a critical flaw. Infantry based AT ordinance was either unwieldy (field guns and AT guns were accurate to range, but you can basically forget about rapidly repositioning them repeatedly in a fight. That shield on it isn't really protecting you either. Bazookas were powerful but had a limited range. We can't actually confirm that the US ever used the "sticky bomb socks" mentioned in field manuals) US armor without support was prone to being destroyed, ect ect ect) or lacked for range, and tanks were never intended to not be supported by infantry. There was plenty of ways of dealing with tank that didn't involve other tanks- a single well placed molotov could render a tank inoperable.
Frankly when discussing the success of one army against another in WW2 in the European Theater you need to be keenly aware that the Germans were lying and manipulating statistics. You're free to access why they were doing this on your own, but needless to say they were extremely hesitant to report their own losses and extremely quick to over-estimate their successes. Tanks well behind the enemy lines for German statistics could often be reported as being "down for repairs." German high command was often completely unwilling to acknowledge defeats and short comings. Germans were also extremely prone to over-reporting successes and it is well known that many German tank aces were intentionally awarded credit for their entire platoon or company. German air aces were more a story of policy than anything else- German Luftwaffe flew till they died or physically could not fly. Most all the allies retired their airmen to training roles or pushing paper once they hit ace status. Furthermore the Allies were far quicker to report their standings. Vehicles that were not immediately ready for use were recorded as losses. A single T-34 or M4 Sherman could be recorded as a loss 5 or 6 times if it was prone to getting stuck in the mud, or needed to have it's engine overhauled, it's transmission replaced, ect. German tanks which would, statistically, spend more time in the shop (they weren't necessarily less reliable, but they were definitely more maintenance hungry. Even the Panzer 4.
US operational doctrine was more than adequate. The idea that tanks were supposed to support infantry wasn't entirely true, but everyone was keenly aware that infantry- or tanks- that didn't have support from other army elements weren't going to last long in the field was something everyone in leadership was aware of. The fine documentation of the tank destroyer doctrine was conceived to deal with Blitzkrieg tactics (this in itself wasn't exactly a thing: http://forum.worldoftanks.com/index.php?/topic/198175-mythbusters-blitzkrieg-decoded/ it wasn't the fact that the Germans had tanks and aircraft and could use them together that made them so capable, it was the fact that they had very good communications down the line, and very good engines to match their vehicles. The folks the Germans were fighting were still stuck in a WW1 organizational and logistical mindset, which would prove to be disastrous for the French) which ran in the idea that as the Germans pushed through US lines, tank destroyers- this is why the US was the only country to employ turreted tank destroyers with, typically, light armor that were very mobile- would function as "fire fighters" moving from hot spot to hot spot.
As it were, the US were more than capable of dealing with German tanks without the aid of dedicated tank hunting units. While the M10, M18, and M36 were all capable tank destroyers the fact of the matter was that the 76mm M4 Sherman, and the 75mm M4 Sherman were in their times perfectly adequate tank fighters. While the US didn't really end up following it's tank destroyer doctrine, it wasn't exactly necessary either. At some point the M4 Sherman may have been conceived as an infantry support tank but it was more than capable of fighting tanks.
http://ftr.wot-news.com/2013/07/28/please-dont-use-the-5-m4s-1-panther-myth/
http://ftr.wot-news.com/2013/08/03/cheating-at-statistics/
http://tankarchives.blogspot.ca/ (particularly, pay attention to the series on "cheating at statistics"
Albert Speer's Inside the Third Reich
Tank destroyers were used by all the major combatants in WW2. The German Army had the Jagdpanzer IV, the Jagdpanzer 38(T), and the Nashorn. The USSR had the dual role SU-76, the SU-85, and the SU-100. The US fielded the M18 Hellcat, the M10 Wolverine, and the M36.
Properly handled as part of a combined arms doctrine, tank destroyers used by the US were fearsome tank killers. In July, 1944, the 630th Tank Destroyer Battalion reported knocking out 53 Tigers and Panthers and 15 StuG IIIs with a loss of just 17 M18 Hellcats. As the war progressed in Europe, the Germans were forced to attack or defend with just tanks, no real air support, and few infantry and artillery units to help. Tank destroyers lived for that kind of battle and both the USSR and the US took full advantage.
With an open top and lighter weight, the TD could move and reposition faster, go longer distances, hold more ammunition than a tank, cost less to build, and was easily repaired. The term "shoot and scoot" was given first to TD. Over 10,000 motorized TDs were built and fielded by the US alone.
Tank destroyers were part of the highly effective US combined arms doctrine: infantry, scouts, tanks, artillery, forward tactical air support and spotting, and finally tank destroyers were used together to form a complete and effective warfare platform.
While tank destroyers had a strong role to play in WW2, it was strictly an interim solution, much like the Germans using their antiaircraft 88mm guns as tank killers. The larger, more powerful tanks that were being fielded as WW2 progressed were too heavily armored to be tackled by the tanks and fielded artillery of the time so tank destroyers took over that role. General Patton predicted that tank destroyers would become unnecessary as tanks could simply do the same job once their armament was as good as or better than the tank destroyers. Indeed, towards the end of the war, tank destroyers became less useful and the breed has pretty much disappeared since then.