So I've been rewatching every Marvel MCU movie each night recently, and I've come up with a bit of a theory regarding them and was interested in what others think of this. It all derived from the fact that recently the statements "Phase 1 was great because it all lead to Avengers" and "Phase 2 sucked because AoU could stand alone and no one would know".
Well here's the "theory" of how it all ties together and how it all plays into Infinity War:
~Iron Man; this movie sets up the entire franchise. There isn't a whole lot to tie into the entire franchise other than advanced tech, possibility of doing more, and SHIELD. I equate this movie to being the pilot in a TV show. Introduces some parts, gets you started, doesn't dive too deep.
~Incredible Hulk; it's in there but I kinda disregard. Although it does foreshadow Captain America a bit as it seems the whole movie centers around getting the tech that Banner knows about.
~Iron Man 2; this starts to get into the juicy bit of the MCU if you ask me. It introduces the depth of SHIELD and how far it reaches and how far it goes back. It also expands on the fact that despite Tony's vast intelligence, there are others in the MCU who may be just as smart as he is. But his advanced understanding give him a one up.
~Thor; Sets up the fact that there is much more out there than just earth. It shows a grander scale of the universe that the MCU sits in (especially the '9 realms'). Thor also brings forth the idea that humanity is woefully behind other races when it comes to protecting itself. Other than that fact, Thor's Phase One movie has very little overall impact.
~Captain America: The First Avenger; This is the one that kicks forward the Avengers. We learn about the real power of the tesserect. It also introduces Hydra, who plays a much bigger part in all of this than I think most assume, and later in my theory I will address it.
So where where it gets interesting to me: From the Incredible Hulk, it pulls the idea that Captain America can't be recreated, and in a way that makes him all alone. From Iron Man 2 it obviously pulls SHIELD, it's reach, and what it can do. But it also pulls that it needs Tony Stark, because his intelligence goes one step beyond many others in the MCU. From Thor we obviously get our Villain, but we also get that Loki picks earth because of how unprepared we are to battle many of the other forces in the 9 realms. But we used the tech from Thor to essentially help arm us. From CA comes the tesserect.
So in phase one the avengers pull a bit of each movie to make it possible that all the movies tie in and in a way that the movies aren't just origin stories for characters, but for parts of the universe as well.
Ok so phase 2 gets a little more hairy because I don't think that you can sit there and watch in order one time and understand how it all plays in.
~Iron Man 3; Seems like a fun romp buddy com movie, but here's how I think I goes much deeper. It introduces "Extremis", which I might have through up a link to in AoU. But it also intros the idea that (1) Jarvis is a little more independent operating that thought of, and (2) it brings forth the idea that protecting the planet might be a little much for the Avengers, and an autonomous defender might be the answer.
~Thor: The Dark World; brings forth another infinity stone. Other than that, it has very little impact on the AoU.
~Captain America: The Winter Soldier; The fall of SHIELD, which means there are still many parts of SHIELD scattered all over the place. It also shows how far SHIELD has gone and the way it lost it's ideals from being a clandestine entity protecting the planet to a spy agency that wasn't protecting anything.
~GotG; No overall impact on AoU, but it plays into the bigger picture of the Avengers with another Infinity Stone being introduced. It also shows again the span and reach of the MCU universe by bringing in more realms that will play a part.
~Agents of SHIELD; It really ties all parts of the universe together. It shows how after the fall of SHIELD, that Coulson and his team were there to clean up the parts. What the show hints at for almost a full year is that they are collecting the secrets of SHIELD that Hydra stole in the chaos. Behind the scenes they had been secretly using the Avengers to collect all of this to bring it all back into protection.
~AoU: So to me the small clues from the previous movies that it plays a part are from IM3, Ultron is from Tony seeing how Jarvis controlling all the suits could have made Tony someone who didn't need to enter the fight. I also believe that Extremis plays a part int the cradle that helped heal Hawkeye and build Vision. Thor DW established that Thor is always there to protect the earth and that he was there to protect it from the mess of SHIELD/Hydra. From WS and AoS, we get that the start of AoU was the Avengers cleaning up the mess left from the events of WS. That clean up of WS lead Tony to the Mind Stone, which is what helped create Ultron.
So now this next bit takes from still watching AoS:
Hydra is still well and alive, but it's revealed that Hydra's true world domination comes from worshiping Hive and everything that they did comes from bringing him back to earth.
How here's where I really play into a theory of looking forward:
I think that CA:CW, DS, GotG2, Thor: Ragnarok, Black Panther are all going to be about them more being on their own in learning the ties to a greater universe out there. There are other realms, but I feel as though Thor, who knows the most, hasn't shared everything with the Avengers. CW is going to leave the heroes scattered. Doctor Strange will introduce the mystical to MCU, but more than just that, I think it's going to show a connection to a realm that hasn't been brought forth in the MCU yet. GotG2, I feel, is going to bring the focus to how the GotG come to earth and tie into the Avengers. It's going to be the drawing force of what really brings Thanos to focus on earth more than the war of New York in the Avengers. Thor is going to really have to do with all the stones coming together. There's no way around it, Thanos is going to be a big player in this movie and it's going to be the driving force to IW Part 1.
So we also know that all along Thanos has been watching these events unfold:
My theory with this, Thanos found Hive, and created Hydra. Thanos is the one that created conflict between the frost giants and asgardians to put Loki in position to rule over Asgard and create conflict. Obviously his connection to Ronan the Accuser is out there, and it was a way to eliminate the Nova Corp.
I think that everything that happened so far in the MCU was strings pulled by Thanos, to make his ability to control every one of the 9 realms easier by having so much conflict on each realm that a strong and powerful leader like Thanos was welcome.
So have at it, just be gentle