"Let us on that day rededicate ourselves to the
nation, "one and inseparable" from which every thought that is not
worthy of our fathers' first vows in independence, liberty, and right
shall be excluded and in which we shall stand with united hearts, for an
America which no man can corrupt, no influence draw away from its
ideals, no force divide against itself,-a nation signally distinguished
among all the nations of mankind for its clear, individual conception
alike of its duties and its privileges, its obligations and its rights."
Woodrow Wilson could not have picked a better time to try and unify the country through the symbol of the American flag. On May 30th, 1916 he gave his Presidential Proclamation 1335, which established Flag day. He had many motives for doing so. The United States had become a nation of immigrants in the second half of the 19th and early 20th century, there were still deep cultural divides between North, South, and West, and the country was again being pulled apart by the European conflict later known as WWI. Wilson won the presidency on a pacifist platform and continued to keep the US out of the international war. However, the Lusitania had been sunk and continuing German u-boat threats to the seas prompted many Americans to advocate the country joining the war. Flag day has lasted but Wilson's pacifism did not and reluctantly followed with Congress's declaration of war less than a year later.
"Many circumstances have recently conspired to
turn our thoughts to a critical examination of the conditions of our
national life, of the influences which have seemed to threaten to divide
us in interest and sympathy, of forces within and forces without that
seemed likely to draw us away from the happy traditions of united
purpose and action of which we have been so proud."
Woodrow Wilson's Complete Speech from the American Presidency Project
http://www.presidency.ucsb.edu/ws/?pid=62991