Trump failed to recognise the causes of the recent unrest, and has not been strong enough in his criticism of the role played by the police and right-wing groups. In only criticising Black Lives Matter protesters, Trump has shown he has a poor understanding of racial issues, and this means he will not be able to relieve tensions.
The use of greater police powers to restore law and order might be effective in bringing an end to the protests in the short term, but it will not address the underlying issues of structural racism, inequality and discrimination that are the root causes of the unrest.
Trump might be able to reduce the surface-level violence, but race relations run deeper than this. Trump only recognises the symptoms, not the disease.