My thoughts are that Trump will try Syria/Lybia but when he finds out that the Kurds don't want to march on Tehran, he will pivot to Irak 2... and soon find himself in Afghanistan scenario.
China will be delighted to have the US weaken itself economically and militarily in another quagmire and use the diplomatic repercussions to strengthen its position in Africa and its alliance with Pakistan.
Russia will help Iran in the same way that the US has helped Ukraine. Enough to help them last, not enough to let them win. All while profiting from expensive oil prices and better relations with China.
The long term consequences for the US will be similar to Vietnam: economic crisis, bankruptcy and debt reestructuring. Instead of isolating China, they will reinforce its diplomatic position, while at the same time weakening the rationale for its own "defensive" alliance.
