From the very beginning of Agamemnon, it's clear that the people of Argos are living in the past. If you were a Watchman who had to spend every night of your life for ten years sitting on a roof in the cold waiting for a sign that your king was coming home, you might start thinking that it was time to move on, right? When the Chorus first comes on stage, we see that they are suffering from the same symptoms; ten years later, they can't stop talking about how the Trojan War first got started. When it turns out that Agamemnon is coming home, it seems as if the past is going to be relived in the present; you can already see a hint of this idea when the Watchman says he can't wait to shake Agamemnon by the hand. As it turns out, the past is going to come back, but not the past everyone was hoping for. Instead, it is the past of murders in the previous generation (the children of Thyestes killed by Atreus) and on the way to Troy (Iphigenia sacrificed by Agamemnon) that will come back to plague Agamemnon and Cassandra. Thus, the theme of "Memory and the Past" brings us full circle to our initial trio of main themes: "Justice and Judgment," "Fate and Free Will," and "Revenge."
Agamemnon acts as if the past is no big deal; this, however, is ultimately self-serving.
The past in Aeschylus's play is more important than the action we see onstage, in the present.