There mere presence of the military is dangerous because it creates a precedent. These precedents will me abused, perhaps not by Trump, but someone will push the boundary further. The "slippery slope fallacy" isn't a fallacy when it comes to power base don precedent.
The urban crime rates are in large part an unintended consequence of the war on drugs creating an environment where black markets and violence thrive. Additionally, the regulatory system enforced by police reduces economic opportunity with red tape and artificial financial obstacles. The police have created the problem they say they can't police.
Democrat complaints about politically-motivated prosecutions certainly do ring hollow.
Deploying the national guard in U.S. cities, even because of high crime rates, is hardly precedent setting. The same thing was done in 2006-2008 and at other times. I guess it's a reasonable question to ask, how bad does crime have to be before it crosses the line into unrest (which is where historically the National Guard has come into play)? 1 murder a day? 10? 100? 1000?
The war on drugs may be part of the problem but I don't think that's the whole story. Especially since that once you get to drugs harder than marijuana (heroin, fentanyl, crack, etc.), I don't think relaxing enforcement (at least in terms of dealers) would be a net benefit to society. Besides, I don't think the war on drugs, which started in the 1980s, is responsible for recent crime increases. On the other hand, Trumps national guard deployments don't match exactly where crime has been increasing in all cases (I'm thinking specifically of Chicago where crime, while high, has generally been on the downtrend for a while). However, Memphis and Washington D.C. where the National Guard was deployed are cities where crime was already high and in recent years gotten worse.