What Was the Sussex Pledge?:
The Sussex Pledge was a promise given by the German Government to the United States of America on May 4th 1916 in response to US demands relating to the conduct of the First World War
What Did It Pledge?:
Germany promised to alter their naval and submarine policy of unrestricted submarine warfare
and stop the indiscriminate sinking of non-military ships. Instead, Merchant Ships would be searched and sunk only if they contained contraband, and then only after safe passage had been provided for the crew and passengers.
Why Was The Sussex Pledge Issued?:
On March 24th 1916 a German submarine in the English Channel attacked what it thought was a minelaying ship. It was actually a French passenger steamer called 'The Sussex' and, although it didn't sink and limped into port, fifty people were killed. Several Americans were injured and, on April 19th, the US President (Woodrow Wilson) addressed Congress on the issue. He gave an ultimatum: Germany should end attacks on passenger vessels, or face America 'breaking off' diplomatic relations.
How Did Germany React?:
It's a huge understatement to say Germany didn't want America to enter the war on the side of her enemies, and the 'breaking off' of diplomatic relations was a step in this direction. Germany thus responded on May 4th with a pledge, named after the steamer Sussex, promising a change in policy.
Did The Sussex Pledge Last?:
No. As the war raged on in 1916, the German High Command became convinced that, not only could they break Britain using a full policy of unrestricted submarine warfare, they could do it before America was in a position to fully join the war. Consequently, on February 1st 1917, Germany broke the Sussex Pledge and returned to sinking all 'enemy' craft. These actions contributed heavily to America's declaration of war on Germany, issued April 6th 1917.
President Wilson comments on the Sussex Incident:
"...I have deemed it my duty, therefore, to say to the Imperial German Government, that if it is still its purpose to prosecute relentless and indiscriminate warfare against vessels of commerce by the use of submarines, notwithstanding the now demonstrated impossibility of conducting that warfare in accordance with what the Government of the United States must consider the sacred and indisputable rules of international law and the universally recognized dictates of humanity, the Government of the United States is at last forced to the conclusion that there is but one course it can pursue; and that unless the Imperial German Government should now immediately declare and effect an abandonment of its present methods of warfare against passenger and freight carrying vessels this Government can have no choice but to sever diplomatic relations with the Government of the German Empire altogether.
This decision I have arrived at with the keenest regret; the possibility of the action contemplated I am sure all thoughtful Americans will look forward to with unaffected reluctance. But we cannot forget that we are in some sort and by the force of circumstances the responsible spokesmen of the rights of humanity, and that we cannot remain silent while those rights seem in process of being swept utterly away in the maelstrom of this terrible war. We owe it to a due regard to our own rights as a nation, to our sense of duty as a representative of the rights of neutrals the world over, and to a just conception of the rights of mankind to take this stand now with the utmost solemnity and firmness..."
Cited from http://www.lib.byu.edu/~rdh/wwi/1916/sussex.html at The World War One document archive. Exerted from United States, 64th Cong., 1st Sess., House Document 1034. 'President Wilson's remarks before Congress concerning the German attack on the unarmed Channel steamer Sussex on March 24, 1916'.