I've been studying a lot about WWI and am often surprised by the casualty numbers. I understand that once the battle were on open ground tanks and aircraft were very good at breaking defenses.
My main question is why invasions like Normandy and the amphibious assaults in the pacific, were so successful for the allies. They often suffered heavy casualties, but nothing compared to what battles like this in the previous war. When landing on the beaches the troops wouldn't have the tanks available to help them break the machine guns and artillery, which would have been prepared for these landings. What sort of tactics did they use to not get slaughtered in the same way troops did 20 years before?
The weaponry isn't the deciding factor here for small units. The rifle that everyone bar the USA took into WWII as standard was essentially the rifle they concluded WWI with, and a lot of other weapons are the same too. The Vickers machineguns taken by Britain into WWI stayed in service into the 1950's, and both Germany and the USSR made use of large numbers of Maxim-style WWI era machineguns (though German propaganda plays up the ubiquity of the MG-34 early in the war) In fact, the UK, Germany, France and Soviet Union issued WWI rifles to soldiers during the war, and the US Marine Corps was using the M1903 Springfield (the official standard rifle and second most common US-used rifle in WWI) in large volumes during the early battles of the pacific war due to Army priority in re-equipping with the M1 Garand.
If you want to understand the shift in warfare between WWI and WWII you need to look at fewer guns and more books. Specifically, the work of interwar theorists such as Basil Liddell Hart, JFC Fuller, Heinz Guderian and soviet thinkers like Tukhachevsky and Svechin, who spent 1918-1939 developing military theory and adapting to the new technological environment. The stalemate of WWI largely comes from the ability of the new technologies of the era to be more easily applied to defence in the current military framework than attack. As WWI progresses we see this deficiency addressed as things like the German Stormtroopers, British assault tactics and the beginnings of combined arms warfare start to appear and armies adapt to the new technological conditions, though limited by the frailty of some of their new technology.
By WWII there are three important factors that have changed:
During WWII the USA produced a staggering amount of equipment, the least interesting but some of the most important being trucks and transport vehicles numbering in the hundreds of thousands. As a result the US army (and vast majority of the armies of its allies via lend-lease) had both motorised combat formations and supply lines. Horse-borne infantry are limited by the mobility of the horses that pull their weaponry and supplies, being limited pretty much to the speed of laden horse, some number of miles per day. Motorised infantry are limited to the speed of their transports - some number of miles per hour. This allows movement both of attacking troops and reserves at much greater speeds, both of which allow more rapid attacks and counterattacks and restore a level of mobility to warfare. The frontline fighting is still very bloody, but it's now possible to move once you get through that frontline and stay supplied if you do. Compare Operation Michael in 1918, where the German army successfully breaks out towards Amiens but loses momentum as supplies can't keep pace with the troops to any of the numerous breakouts and subsequent rapid movements of WWII: the encirclement of Stalingrad, the fall of France, the whole Desert theatre.
The new technologies of WWI weren't 'done', they were ready to use but had not yet reached maturity. Just on raw technical statistics the dfference between a MkIV Landship and a Panzer III are massive. In speed, armour and more the Panzer III trounces the MkIV. However what's almost more important is that the Panzer III works, both mechanically and ergonomically which is a massively overlooked factor of military technology. The engine doesn't vent fumes into the crew compartment, the commander can see out via an all-around cupola, the armament is mounted in a 360 degree rotating turret with a wide range of fire and the crew (though not as comfortable as some other vehicles) are not routinely rendered unconscious by the act of operating their vehicle for any length of time beyond about 45 minutes.
These vehicles are also available in vast numbers to both sides, as a rough estimate about 2000 tanks were produced by Britain in WWI, in WWII the UK managed ~78,000 tanks and SPGs and Germany ~68,000. Both smaller figures than the US and USSR.
This also goes for aircraft too. Compare the operational range, reliability, speed, flight ceiling and armament of German Biplanes to the ME-109, or of british biplanes to the Spitfire and Hurricane, and also note that these aircraft have enclosed cabins with oxygen supply for high-altitude flight, radios so that they can operate as a group or in concert with ground forces. Then add the fact that these aircraft were mass-produced and available for combat in large numbers.
Part of the big problem with WWI was that a lot of the new tools and their influence had not yet been incorporated into the overall body of military thought. Whether it be tanks, airpower or man-portable machineguns a weapon is of limited use until those equipped with it know how to use it to forward the goals of the overall armed forces.
It may seem obvious to us now that tanks are best employed on the offensive to break weakspots alongside supporting infantry who follow up, but this isn't immediately obvious. What if the men get in the way? What if the tanks outrun them? What if they end up taking friendly fire from the tanks? What about if tanks are just better alone unburdened by squishy meat soldiers who have to care about machineguns? This is all the kind of thing that requires assessing, training for and ultimately a little bit of trial and error. WWI was that trial and error. It's a bit of a propaganda piece in favour of tanks, but for this understanding compare the early British employment of tanks in 1916 (throw them in, I'm sure they'll be fine with the mud!) to the understanding shown by interwar theorists like Guderian in Achtung! Panzer where he explicitly states that some terrain is just not good for tanks and that they should be committed in terrain where they won't be hamstrung.
There is a lot more in the way command and control were managed and the intricacies of the tactics and strategy involved, but I'd prefer to leave that to someone who'll get the details right.
When landing on the beaches the troops wouldn't have the tanks available to help them break the machine guns and artillery
Sometimes, but often those tanks were in the first or second wave. The most famous are the DD tanks: put a canvas screen up around your tank and eventually it will float. Sometimes it worked and sometimes it didn’t, but tanks were usually on the beach by the end of the first day regardless.
However, take a closer look at the WWI battles. Most of the time there was a significant initial breakthrough, but the attackers couldn’t keep up the pace, the defenders rallied behind new defenses, and the offensive stalled. But even without tanks and against very strong defenses the attackers could break through initially, if only for a few days or weeks.
So it was with an amphibious landing in WWII. Even without tanks the attackers could break through the enemy defenses, though it may be costly (Omaha is the perfect example as the DD tanks mostly sank). But once you have that beachhead and you’ve taken the bunkers overlooking the cliffs, subsequent waves will land tanks. These will directly support the push inland and reduce infantry casualties. The abundance of trucks and other motor vehicles landing in D-Day+X will allow the supplies to make their way to the front. Indeed without tanks Normandy would have bogged down in the bocage and around Caen becoming very similar to WWI, and without the Mulberry Harbors, undersea oil pipelines from Britain, and the Red Ball Express the advance across France would have taken far longer. You can still make major advances with mediocre tanks and logistics, see the Japanese in Thailand and Malaya in December 1941, but those were the main reasons the Allied invasions did not stop moving forward and eventually get pushed back no matter how tough the defenses were, even if the advance was at a snails pace at times.