George Washington called the Congress under the Articles of Confederation a "shadow without substance" because under the Articles of Confederation the Congress was given very limited powers. The Articles of Confederation gave most of the power to states and did not give much power to the national government. Therefore although it created national bodies like Congress they were just a shadow because they had very limited powers and not much of an ability to institute any real policies.
Most countries dont need "justification" for joining a war, typically the only justification is a decision backing up allies, and under this definition, the US was justified, Japan had also initiated aggression againts the US, giving the US an incentive to join the war, so yes, its likely that they were justified in joining the war, for several reasons