The US mobilised 700,000 US troops (and 300,000 coalition troops) just to liberate tiny Kuwait from Iraq in 1991. 900 or 120,000 (or even 500,000) soldiers makes no difference, neither come close to the manpower needed for a serious military operation in Iran. (No coalition forces would be involved in any war with Iran.)
Not to mention the logistics of invading a country 4x the size of Iraq (and with a far more challenging mountainous terrain). In 1980 the US tried to militarily rescue US hostages in Tehran using 8 military helicopters and a transport aircraft. The operation failed miserably due to a sandstorm and the US withdrew, losing 6 helicopters, the transport aircraft and 8 US soldiers died. [A 2002 $250m US military simulation of a US invasion of Iran ended with the sinking of the entire US carrier fleet in the Persian Gulf, with 20,000+ US soldiers killed in the first day. The simulation was stopped and the general in charge of Iran in the simulation was instructed that he couldn't do what he did. The simulation was thus restarted to ensure a US victory.]
The ICJ recently declared that US sanctions against Iran impede humanitarian trade with Iran (of basic medicine and food), and ordered the US to lift sanctions to rectify this. The US rejected this and instead expanded their illegal extraterritorial sanctions against Iran.
Iran has fully complied with the JCPOA/Iran nuclear deal, the US has violated its obligations under the deal by reimposing sanctions despite the IAEA certifying Iran's compliance on numerous successive occasions (we're in double digits now). Now the US sends more soldiers to the Persian Gulf region and threatens Iran with war!
Regardless, this is something pushed by deep state elements within the US, the military-industrial complex (Trump basically admitted this a few days ago in an interview) and Israel and Saudi Arabia, who want US to fight Iran down to the last American. Trump doesn't want war and the Pentagon knows that any war with Iran cannot be won. Iran has been preparing for this for decades.
The US spent $6 trillion to wage war in Iraq and Afghanistan, and what did they achieve? The creation of ISIS, the creation of the PMU and the shift of Iraq into Iran's sphere of influence, and the Taliban again control most of Afghanistan and the US is desperate to negotiate with them to exit Afghanistan after 17 years.
If the US decided to launch an illegal war against Iran this would be the end of the US.