They [already did this](https://lb.usembassy.gov/alert-u-s-embassy-beirut-lebanon-september-24-2024/), and have been evacuating US citizens for 2 weeks now. And "issue an evacuation order" isn't how this works, the state department, through the embassy (in this case) issues an evacuation notice when they feel there is time and wherewithal for citizens to leave. The US president doesn't force American Citizen's to leave another country during a state department evacuation order.