The threat to the USA is minimally military: it's cultural, economic and partisan.
It's like giving feuding children some knives and guns in lieu of borough reforms and trade agreements... then accusing one child as "a bad one/a threat". The query of "Iran's threat" requires geopolitical/social context. Let's not forget that Chevron, Exxon and Emerson made bids for Iraq's oil fields and that the US spent 3 trillion on a war on Iran's neighbor and stoked global Wahhabi regimes.
Iran is the ace of spades in the republican "foreign threat" news system, whereas Russia is favored by the Democrats.
If there is peace with Iran, the Republicans lose their media story of "foreign threat", whereas the Democrats see a peaceful Iran as the key to stability in the Orient. The US needs to attack Iran for its own internal political media.
Iran has armed Kurds and Yemenis (poor minorities), and fomented Hesbollah. The US has spent 3 trillion dollars on war, asymmetrically armed all of the Orient, encouraged global wahhabism via Saudis, encouraged Israel. The USA has ignored the development of the region's middle class, culture and education.
Saudi Arabia poses more of a threat to America, as demonstrated by Wahhabi actions of 9/11, Isis and Afghanistan, whereas Iran has moved to pacify ISIS and has no choice but to combat the threat posed by the USA and Saudi Arabia.
With 3 Trillion dollars of military action and institutional debasement that have stoked 15 million refugees onto Iran and Europe, I'd use irony to parallel your question: "What threat does a child pose to their sodomist?", with the United states being an abusive presence in the backwards and treacherous family of nations of the Orient. ISIS versus Hesbollah is an illustration of the analogy.
The USA represents a good ally for Muslims as a potential relief from division and violence. The US can pacify the Orient by obliging trade agreements in between Iran and SA and modernizing Iran to help it achieve a middle class and a peaceful transition.
The USA, Israel and Saudi Arabia's escalations of violence are a regional threat in the middle east and to all of Europe, while Iran has pacified ISIS and created a more favorable climate for Exxon, Chevron and Emerson to operate in Iraq.
By destabilizing Iraq and Libya and strengthening Saudi Arabia, the USA debases a house of cards which causes millions of refugees to flee to Europe. The USA is a threat to itself more than Iran, and to the EU.
The multi-trillion sponsorship of radical ideologies like Saudi Arabia fanaticism and Israeli ownership of Golan causes tangible violent results when paired with a 3 trillion war against Saddam, compared to Iran's ideologies which have more in common Turkish nationalism than global wahhabism.