There is a wealth of interesting issues on this broad topic, but since it would take a book to treat these issues in detail, here is just a sample of some of the more interesting questions. Consistent with its isolationist inclination, the U.S. declared its neutrality early in the conflict and managed to stay on the sidelines until April of 1917. But what actually caused the US to enter the war? The answer most often given is that Germanyís declared policy of unrestricted submarine warfare compromised American trade interests enough to bring us into the war. Germany had begun and then backed away from such a policy during the early years of the war, in the face of strict protests from the U.S. and other neutrals defending their right to continue to trade despite the war. When Germany finally declared it would unleash its subs to attack any and all ships without surfacing to search the ship and protect the crew, the U.S. joined the war against Germany. However, the reason for U.S. intervention is not that simple. The public outrage over discovery of the Zimmerman telegram certainly made it possible for Wilson to lead the country to war, and one must wonder whether the public would have supported sending its boys to a distant war simply to protect its commercial interests, absent the direct threat on the integrity of the U.S. the telegram provided. It is at least possible, if not plausible, that the motivation behind the U.S. entry into the war was more a realization that it did not want a German-dominated Europe, rather than a pure objection to German violations of American neutrality.
Other interesting questions include the following. Was Germanyís use of submarine warfare really illegal, or was international law incapable of adapting to the new technology of the war? Was Britainís blockade of the Central Powers a legal measure (with its extension of the contraband lists and its failure to place its ships immediately around the blockaded area), and was it ever imaginable that the U.S. protests against British violations of its neutral status would have led the U.S. into the war against Britain? And as to U.S. neutrality, did the U.S. fully respect its neutral status prior to 1917? Did it violate neutrality to sell arms to the Allies on credit?
Answers to all these questions seem to
indicate that, consistent with the conclusions of my paper, where the legality
of a certain action was unclear, countries nonetheless acted in their national
interest, often bending or breaking the international legal rules in existence
at the time.
Law of the sea and submarine warfare
For the 1907 Hague Convention Relating to the Conversion of Merchant Ships into War-Ships (which entered into force on 26 January, 1910), see http://www.lib.byu.edu/~rdh/wwi/hague/hague8.html. See also the Hague Convention Concerning the Rights and Duties of Neutral Powers in Naval War (same entry into force), http://www.lib.byu.edu/~rdh/wwi/hague/hague12.html. Among other things, this convention forbade both the capture and the exercise of the right of search by belligerent war-ships in the territorial waters of a neutral power as a violation of neutrality. It also established some procedural guidelines for the establishment of prize courts within neutral states. There are too many other provisions to summarize here, but click on the above link to see all 33 articles of the convention.
http://www.lib.byu.edu/~rdh/wwi/1914/armship.html gives the U.S. State Department statement of September 19, 1914, on the status of armed merchant vessels arriving in U.S. ports. It attempts to distinguish armed merchant vessels which constitute ships of war from merchant vessels merely carrying arms for defensive purposes. The statement provides a list of factors to indicate that munitions will only be used defensively, but in the end, even the list is not conclusive, and each determination was to be made individually, weighing all the relevant evidence. This statement indicates the trickiness of separating the belligerent ships from the neutral ones.
President Wilson's gave his first warning to the Germans on 10 February, 1915. See http://www.lib.byu.edu/~rdh/wwi/1915/strict.html. Wilson reminds the German government that a belligerent's only right in dealing with neutral vessels is to visit and search, unless an effective blockade is implemented. Germany's proposed policy (http://www.ukans.edu/~kansite/ww_one/naval/dc150204.htm) of attacking a ship in the waters around Great Britain without first identifying it as a belligerent or discovering contraband cargo would be, according to Wilson, an unprecedented act of naval warfare. Wilson then defends the consistent neutrality of the U.S. and ends by saying that if Germany would destroy the lives of Americans on the high seas, the U.S. would view the action as an indefensible violation of neutral rights and would hold Germany to strict accountability for these acts.
For an excellent site on the use of submarines during the war, go to http://uboat.net/history/wwi/index.html.
Submarines of the Great War, http://www.dropbears.com/w/ww1subs/index.htm,
provides a series of photographs of subs from the war, as well as a unique
recount of a 1917 Allied submarine mission to immobilize the main German
U-boat base at Bruges, Belgium.
Neutrality of the United States
On August 19, 1914, President Wilson declared to Congress that the U.S. would remain neutral despite the European hostilities. See http://www.lib.byu.edu/~rdh/wwi/1914/wilsonneut.html. The text of the speech focuses primarily on the attitude that Americans will take to the war, given that many Americans had come from the belligerent nations. Wilson therefore calls upon all his countrymen to remain neutral in fact as well as in name, stating that they must not allow partisanship to lead them to take sides in the conflict, because this is the "deepest, most substle, most essential breach of neutrality."
http://www.pbs.org/greatwar/interviews/winter9.html is a brief description of the significance of the German sinking of the Lusitania. Jay Winter, a Cambridge University professor, explains the logic of having to shoot down merchant ships: the German navy decided to interdict war supplies going from North America to Britain, and almost any ship could conceivably be carrying munitions; thus the Lusitania posed a threat to the German war effort. This "total war" could not stop to worry about civilian casualties.
For a look at the Zimmerman telegram, go to http://www.lib.byu.edu/~rdh/wwi/1917/zimmerman.html. In the January 19, 1917 note, Zimmerman, the German Secretary of State instructs the German minister to Mexico to propose to the Mexican President an alliance with Mexico if the United States enters the war. Germany promises to give its financial support and suggests its assistance in Mexico's reacquisition of New Mexico, Texas, and Arizona.
For a brief timeline of America's march to war, see http://www.worldwar1.com/dbc/march.htm. This site tells the story through the use of newspaper headlines between February 1, 1917 ("Germany Begins Ruthless Sea Warfare") and May 19, 1917 ("President Calls the Nation to Arms"). See also President Wilson's War Message, outlining his reasons for entering the war after remaining neutral for the first three years of hostilities.