Securing border under the present administration in the USA

The United States Government is striving to have a "secure" border under the present administration. What would be the consequences/impact to the United States if the border was suddenly made less open and more "secure"? This is a strategic security question and not a political one. What is the role of the Department of Defense in border and coastal security? What is the DOD's relationship with the Department of Homeland Security as it pertains to border security? Why was there such an increased?