The assassination of Archduke Franz Ferdinand of Austria on July 28, 1914 was the casus belli for World War I. The United States joined in 1917:

“Joint Address to Congress Leading to a Declaration of War Against Germany (1917)”

On April 6, 1917, Congress approved a resolution declaring war with Germany. The Senate approved the resolution by a vote of 82-6 on April 4, 1917.

By April 4, 1917, though, it already seemed clear to many that the United States’ entry into the war was inevitable.

[…] I wish to feel that the authority and the power of the Congress are behind me in whatever it may become necessary for me to do. We are jointly the servants of the people and must act together and in their spirit, so far as we can divine and interpret it. […]
“Text of President Wilson’s Address to Congress.” N.Y. Times. February 27, 1917.

WASHINGTON, March 18.—With the announcement of the ruthless destruction of three unarmed American merchant ships by submarines, it was unofficially admitted here tonight that virtually a state of war exists between the United States and Germany.
Technically the United States remains in a position of armed neutrality. Whether this shall be changed before April 16, the date fixed for a special session of Congress, the war-making branch of the Government, President Wilson has not decided. […]
Associated Press. “May Call Congress at Once.” N.Y. Times. March 19, 1917.

WASHINGTON, March 20.—At a meeting of the Cabinet this afternoon opinion was practically unanimous that President Wilson should call Congress into extra session without delay to consider the situation produced by the wholesale sinking of the American vessels by German submarines—in other words, to declare that a state of war exists between the United States and Germany. […]
“President’s Advisers a Unit; Executive May Make an Announcement in Next 48 Hours; Session May Begin April 2; Two-Hour Conference Held by Wilson and Cabinet, Discussing All Phases of Situation.” N.Y. Times. March 21, 1917.

Whereas, public interests require that the Congress of the United States should be convened in extra session, at 12 o’clock noon, on the 2d day of April, 1917, to receive a communication by the Executive on grave questions of national policy which should be taken immediately under consideration […]
N.Y. Times. March 22, 1917.

Knowing why President Wilson would be calling for an early extra session of Congress, there were early expressions of support. As the Troy Record recently reprinted in its column of historical items:

SUNDAY, MARCH 25, 1917
As Troy’s National Guard regiment is called back into national service, Mayor Cornelius F. Burns has ambitious plans for patriotic demonstrations on the day Congress convenes for a probable declaration of war with Germany.
The Second New York Infantry, based in Bolton Hall in Lansingburgh since the Troy Armory was destroyed by fire earlier this year, is one of eleven regiments in nine states mobilized by the War Department this afternoon. The Second spent most of last summer on border-patrol duty in Texas during the U.S. punitive expedition against insurgent warlord Pancho Villa.
Congress meets in special session on April 2. In his capacity as president of the New York State Conference of Mayors, Mayor Burns is calling on his peers to schedule patriotic exercises for that day on the model of events planned for Troy.
“The situation that confronts us is a grave one,” Burns writes in a letter published today, “and it is fitting that the Mayors of Cities of the Empire State should set a good example by publicly proclaiming at this time unanimous and hearty support of the Government in any course which it may deem proper to pursue in a matter so deeply involving the rights, honor and interests of our country.”
Burns calls for assemblies of schoolchildren in every public, private and parochial school in New York at noon on April 2, when President Wilson is expected to ask for a war declaration. He suggests a program including the singing of “the Star-Spangled Banner” and “America,” a flag salute and “appropriate resolutions of loyalty and support.” He also recommends “brief patriotic addresses” and the ringing of bells from every church and public building for the occasion.
The mayor tells The Record that he’d like to see his plan go national. “It would be one of the biggest, sincerest and most patriotic demonstrations in the history of civilization,” he predicts.
“100 years ago in The Record: March 25.” Troy Record. March 25, 2017.


Proclamation by State Commissioner of Education—Issued in Accordance With Request of Conference of Mayors—The Program.
State Commissioner of Education John H. Finley has issued the following:
The President of the Conference of Mayors and other city officials of the state of New York has asked my co-operation, together with that of the schools of the state, in a general public recognition of the gravity of the situation which will confront Congress and the nation at noon, April 2. At that moment Congress, called by President Wilson to consider the duty and the danger confronting the nation in the present crisis, will listen to the President’s message and begin its momentous deliberations, which will in large measure determine the future of the republic. Congress has the right to expect patriotic support and true allegiance from every American in this time of crisis. I can only indorse heartily the recommendation which has gone from the Mayors to each city in the state and extend the same recommendation to all the village and district as well as city schools of the state. I ask that you all observe with solemn and sincere respect the exercises suggested or those which will most appropriately express your devotion to our common country.
I join, therefore, in the recommendation of the President of the Conference of Mayors, Hon. Cornelius F. Burns, that at noon on April 2, as Congress is convening and is being addressed by the President, all the children of the public, parochial and private schools be invited to assembly in their respective school buildings and that the following program or one of like character be carried out, viz:
1. Singing of “Star Spangled Banner.”
2. That appropriate resolutions of loyalty and support be adopted by the children, for transmission to President Wilson.
3. The flag salute.
4. Singing of “America.”
District, Village and City Superintendents are also requested to arrange for the assembling of the pupils and teachers in each school, in order that the program suggested or other appropriate exercises may be carried out. The expression of patriotism and devotion upon the part of the youth of the land will be an inspiring example to all our people and will quicken the love which they have for their country and make them think of the service which they should be ready at all times to render. I feel certain that all the school children and teachers of the state will welcome this opportunity to give expression to their love of their own land, their fidelity in time of national danger, and their earnest determination to preserve the honor, integrity and true greatness of our common mother America.
To Deliver Addresses.

The members of the Philip Schuyler Chapter, D. A. R., who will speak in the different schools of this city Monday on “Patriotism and the Flag,” will be Mrs. Herbert Longendyke, Regent; Mrs. S. A. Silliman, Mrs. J. D. S. Giblin, Mrs. W. H. Palmer, Mrs. W. C. Feathers, Mrs. I. G. Braman, Mrs. E. A. Loux, Miss Margaret Freeman, Mrs. Edward M. Oothout, Mrs. Frank Bayer, Miss Edna Holbrook, Mrs. J. F. Fellows, Mrs. A. T. Lincoln and Mrs. C. F. Wilcox. It will be remembered that on February 12, 1908, the Philip Schuyler Chapter presented 196 silk flags to the public schools, providing each class-room with a flag. These flags were presented at a large celebration at the Armory.
Mayor’s Proclamation.

Mayor Burns to-day issued this proclamation:
At 12 o’clock noon Monday, April 2, while Congress is convening in extraordinary session, the pupils of the public, parochial and private schools of this, as well as other cities throughout the country, will assemble in their respective school buildings and carry out a program of exercises expressive of their devotion and allegiance to our country and their confidence in those in whose hands now rests its destiny.
This expression of patriotism, however, should not be confined to school children. Citizens generally should proclaim their hearty and unanimous support of the government in any course which it may deem proper to pursue in a matter so deeply involving the rights, honor and interests of our country.
I would request, therefore, that while the school children are singing “The Star Spangled Banner” and “America,” adopting resolutions of loyalty and support for transmission to President Wilson and giving the Flag Salute, the bells of every church in the city be rung. In churches where there are chimes the national anthem should be played on them.
As a signal for ringing the bells and starting the school exercises, the national salute of twenty-one taps on the fire alarm will be given.
Troy Times. March 31, 1917: 5 col 3.

Mayor Cornelius F. Burns was the son of Troy-born John W. Burns (abt 1838-1881) and Ireland-born Ellen Gorman (abt 1833-1905). John W. Burns was an undertaker, the son of undertaker John Burns (abt 1810-1875) who came to the United States sometime prior to 1838.