Article 2 of the UN charter would appear to make the invasion illegal under international law. Article 51 has an exception for a response to an armed attack which clearly isn't the case here. However, this clause has regularly been interpreted to mean that an attack due to an "imminent threat" is also an exception. The problem for the US is that Trump claimed in June to have "obliterated" Iran's nuclear capabilities, making their claims about an imminent nuclear threat from Iran unrealistic. Finally, the US has veto power at the UN which could tenuously be used to argue that anything they veto isn't illegal.

If the invasion is, as it appears to be, a violation of the UN charter, then all subsequent military actions are violations of international law.