According to the Constitution (remember that document?) Article 1 Section 8 gives the power to declare war to the Congress not the Executive Branch and with good reason. The decision to go to war should not be done lightly or by a handful of individuals who do not have the personal burden of sending a family member or loved one to fight. Congress was explicitly given the huge responsibility for declaring war not only because they best represent the people but also to allow for proper debate on the pros and cons of the declaration and determine the success criteria of the military action.
The rhetoric around Iran consistently indicates a war is going to happen at some point but no one expects there to be a declaration of war when it begins. The US has declared war 5 times since adopting the Constitution and has won every time it did so. The last time the US declared war was World War II and since then has had military engagements in Korea, Vietnam, Cambodia, Cuba, Angola, Panama, Laos, Kosovo, Iraq, Afghanistan and Libya (maybe a few others I left off). Most of our post WWII endeavors did not go as planned either because the country was not fully committed and/or the scope of the mission kept expanding. Once the US and/or Israel attacks Iran, will the mission be complete after their nuclear program is destroyed? Or will we also have to ensure the Ayatollahs are replaced with democratically elected leaders? And what if the former members of the IRGC start a civil war? At what costs are Americans ready to pursue another war?
