The Iraqi parliament has just voted to expell all US troops.
First of all, Trump is not going to do anything. Period.
Secondly, high crimes are crimes or dereliction of duties perpetrated by a high ranking public official. Indeed, as used in the Constitution, it is doubtful that anyone but the POTUS can commit such—certainly not the VP or any of the Senators who have no powers to order injury to someone else.
On the other hand, the order to have Soleimani attacked with a missile likely qualifies as an additional impeachable offense, especially if it conclusively demonstrated that Trump had taken personal offense at some of Soleimani’s comments on social media. Having read that Soleimani sought martyrdom on the battlefield, one might suspect that Trump got suckered.
After all, the impetus for ISIS was to remove the remaing U.S. presence from Iraq, which got talked into letting some seven installations remain. While that is far fewer than the 24 the Pentagon initially wanted, they were too many for a people that want foreigners out. That Trump failed to consult with the Iraqi government now leaves them no choice but to order the invaders out.8