I'll leave it to the military people to address the consequences of that. I'm a political scientist, so I'll address the politics. Normally, in times of an international crisis, the public "rallies" around the president. We call this the "rally 'round the flag" effect, with the key book here being Richard Brody's Assessing the President.
And you can see the effect quite clearly in Gallup trends. Those large spikes in approval that you see in, for example, George W. Bush's approval ratings? 9/11, and then the invasion of Iraq. The spike in his father's approval rating? The previous invasion of Iraq. The condition, though, is that the effect doesn't occur without elite consensus. Why? For that, we turn to good, ole' John Zaller, and the standard text for all things public opinion-- The Nature and Origins of Mass Opinion. According to Zaller, people don't really have coherent beliefs. Instead, they respond to survey questions based on the balance of considerations in their heads at the time. Those considerations are basically just reflections of the messages they receive and accept from elites. If the messages are all in one direction, public opinion moves in one direction because eventually, even those who are resistant to the message are worn down and accept it. That's the short version, anyway. I'm summarizing a book in a couple of sentences.
Basically, when elites agree that we should rally 'round the president, the public does.
Normally, when the president says, "we're invading so-and-so," elites agree that we should rally 'round the president. It doesn't always work that way. And it didn't quite work that way when George W. Bush announced the troop surge in Iraq in 2007. Dubya's approval rating had already fallen, the war there had been going on for a long time, and there didn't seem to be the kind of elite consensus or subsequent rally effect that one might expect if one simply mechanically assumed that war means a rally effect. It doesn't.
And tonight, Trump is going to announce something about Afghanistan. As I said, I'll leave it to the military people to comment on what he announces, and what the consequences might be, but if we are looking for historical parallels here, the 2007 Iraq troop surge seems like the more obvious one in terms of the politics. Trump's approval rating is comparable to Dubya's around 2007, the war in Afghanistan has been going on a long time, and is already unpopular, and the rally effect relies on partisans across the aisle rallying to the president.
Right now, it is virtually impossible to imagine what possible circumstance would get elite Democrats to rally to Trump. After 9/11, George W. Bush's approval rating went sky-high because whatever Democrats thought of Bush, we were faced with an enemy they had to acknowledge was far worse. That's what war does. After Obama came into office, one of the questions that became interesting was whether a 9/11 event would cause a similar rally, or whether the "Obama is a muslim" bullshit circulating among the tea party would prevent such a rally. I leaned towards the latter position due to the proliferation of Obama conspiracy theories, and the unwillingness of even people like John Boehner to shoot those conspiracy theories down.
Turning that around with Trump, though, consider the recent comments by even people like Senator Bob Corker-- a Republican-- acknowledging the fact that Trump is incompetent and unstable. Combine that with the fact that we now know his campaign at least attempted to collude with Russia, and that he tried to obstruct the investigation into the Russia matters by firing Comey, and we have the reverse situation. No matter what the crisis is, can Trump be trusted to handle it?
And the Charlottesville stuff does factor in here. I've finally stopped ranting about it to begin talking about Afghanistan, and the politics of war, but the more Trump antagonizes large segments of the population, the less possible it becomes for them to rally around him in other circumstances.
And this is part of the trap that Trump faces. Let's be blunt about this. Trump wants a war. He wants a war because he keeps failing and losing. He can't "get a win" on anything legislatively, like healthcare. Tax reform is doomed, although a tax cut will happen (more on that later), his administration is in total chaos, the Mueller investigation has him on the run, his approval ratings are abysmal... Trump has only ever really been praised as President when he shot a missile at Syria and dropped a MOAB on Afghanistan. He knows about wartime rallies, and he thinks he's a tough guy. This is his only option. It won't work to get a rally effect, and militarily... I'll let others comment, but at least you have some citations now.