By using the full title of the German government, Wilson most likely hopes to

impart a negative connotation of the German government to support his argument for war.

give clear and concise information, without leaving out important facts, as he addresses Congress.

make himself sound official and trustworthy as he is addressing Congress,

evoke fear in the members of Congress so they will declare war on Germany.

Your question is incomplete because you have not provided the excerpt, which is the following:

On the 3rd of February last I officially laid before you the extraordinary announcement of the Imperial German Government that on and after the 1st day of February it was its purpose to put aside all restraints of law or of humanity and use its submarines to sink every vessel that sought to approach either the ports of Great Britain and Ireland or the western coasts of Europe or any of the ports controlled by the enemies of Germany within the Mediterranean.

Answer:

impart a negative connotation of the German government to support his argument for war.

Explanation:

In 1917, US president Woodrow Wilson asked the Congress to declare war on Germany during World War I. Thus, by naming Germany's official name, the petition to fight against a country becomes more impersonal, as he introduces his opinions and facts to make a more understandable argument for war against Germany.