President Lincoln's 75,000 Volunteers
President Lincoln's 75,000 Volunteers was a call by the President of the United States on April 15, 1861 to summon 75,000 militia to serve for three months following the bombardment and surrender of Fort Sumter. Some slave states refused to send troops against their neighboring states, with the result that most such states also declared secession from the United States and joined the Confederate States of America.
In April 1861 the Regular Army of the United States of America consisted of approximately 16,000 officers and soldiers organised into ten regiments of infantry, four of artillery, two of cavalry, two of dragoons, and one of mounted rifles. These regiments were mostly posted in small forts of company-sized detachments, the majority posted West of the Mississippi River. Following the secession of seven states from December 1860 to the creation of the Confederate States of America in February 1861, many officers and soldiers resigned from the United States Army to join the Army of the Confederacy.
Legal limits 
Until the early 20th century, the US relied on calling out militia and volunteers rather than expanding the regular army. However, there were restrictions on the number of men and the length of time they could serve that the President of the United States, as opposed to a State Governor could call up.
"Sec. 4. And be it further enacted, That the militia employed in the service of the United States, shall receive the same pay and allowances, as the troops of the United States, And that no officer, non-commissioned officer or private of the militia shall be compelled to serve more than three months in any one year, nor more than in due rotation with every other able-bodied man of the same rank in the battalion to which be belongs." – Militia Act of 1795
On March 2, 1799 the number of militiamen able to be called by the President of the United States for a provisional army was limited to 75,000 men.
The declaration by Lincoln read:
BY THE PRESIDENT OF THE UNITED STATES:
A PROCLAMATION WHEREAS the laws of the United States have been, for some time past, and now are opposed, and the execution thereof obstructed, in the States of South Carolina, Georgia, Alabama, Florida, Mississippi, Louisiana, and Texas, by combinations too powerful to be suppressed by the ordinary course of judicial proceedings, or by the powers vested in the marshals by law. Now, therefore, I, ABRAHAM LINCOLN, President of the United States, in virtue of the power in me vested by the Constitution and the laws, have thought fit to call forth, and hereby do call forth, the militia of the several States of the Union, to the aggregate number of seventy-five thousand, in order to suppress said combinations, and to cause the laws to be duly executed. The details for this object will be immediately communicated to the State authorities through the War Department. I appeal to all loyal citizens to favor, facilitate, and aid this effort to maintain the honor, the integrity, and the existence of our National Union, and the perpetuity of popular government; and to redress wrongs already long enough endured. I deem it proper to say that the first service assigned to the forces hereby called forth will probably be to repossess the forts, places, and property which have been seized from the Union; and in every event, the utmost care will be observed, consistently with the objects aforesaid, to avoid any devastation, any destruction of, or interference with, property, or any disturbance of peaceful citizens in any part of the country. And I hereby command the persons composing the combinations aforesaid to disperse, and retire peaceably to their respective abodes within twenty days from this date. Deeming that the present condition of public affairs presents an extraordinary occasion, I do hereby, in virtue of the power in me vested by the Constitution, convene both Houses of Congress. Senators and Representatives are therefore summoned to assemble at their respective chambers, at twelve o'clock, noon, on Thursdays the fourth day of July next, then and there to consider and determine such measures as, in their wisdom, the public safety and interest may seem to demand. By the President:ABRAHAM LINCOLN
Secretary of State WILLIAM H. SEWARD
Secretary of War Simon Cameron's communique to the various state governors 
C A L L T O A R M S ! ! 75,000 VOLUNTEERS WANTED Washington, April 15. The following is the form of call on the respective state Governors for troops, issued to-day: Sir:—Under the act of Congress for calling out the militia to execute the laws of the Union to suppress insurrection, repel invasion, &c., approved February 28th, 1795, I have the honor to request your Excellency to cause to be immediately detached from the militia of your state, the quota designated in the table below to serve as infantry or riflemen for three months, or sooner, if discharged.
Your Excellancy will please communicate to me the time about which your quota will be expected at its rendezvous, as it will be met as soon as possible by an officer or officers to muster it into the service and pay of the United States; at the same time the oath of fidelity to the United States will be administrated to every officer and man. The mustering officers will be instructed to receive no man under the rank of commissioned officer who is apparently over 45 or under 18 years, or who is not in physical strength and vigor. The quota to each state is as follows: Maine, New Hampshire, Vermont, Rhode Island, Connecticut, Delaware, Arkansas, Michigan, Wisconsin, Iowa, and Minnesota, one regiment each; New York 17 regiments; Pennsylvania, 15 regiments; Ohio, 13; New Jersey, Maryland, Kentucky, Missouri, four regiments each; Illinois and Indiana, six regiments each; Virginia, three regiments.
It is ordered that each regiment shall consist of an aggregate of officers and men of 1,780 men.
The total thus to be called out is 73,910 men, the remainder, which constitutes the 75,000 under the President's proclamation will be composed of troops in the District of Columbia.
Reaction and Resistance 
Rather than a call for 75,000 military volunteers from any American state or territory, the two proclamations called for a specific number of volunteers from each state, including slave states in the South that had not yet declared their secession.
Several Northern states communicated enthusiasms. with states such as Indiana offering twice as many volunteers as requested.
By contrast the reaction in the Southern states not part of the Confederacy destroyed pro Unionist sentiment.
Governor Isham Harris of Tennessee stated in a telegram to Lincoln, "Tennessee will furnish not a single man for the purpose of coercion, but fifty thousand if necessary for the defense of our rights and those of our Southern brothers."
Governor Beriah Magoffin of Kentucky declared that they would not send volunteers to a Northern army intent on subjugating their Southern brethren.
Governor John Letcher of Virginia, whose state has been requested to furnish three regiments totally 2,340 men and officers, has stated in the past his intent for his state to remain neutral. He has today replied to Lincoln that since he has "chosen to inaugurate civil war, he would be sent no troops from the Old Dominion."
Governor Henry Rector of Arkansas stated, "The people of this Commonwealth are freemen, not slaves, and will defend to the last extremity their honor, lives, and property, against Northern mendacity and usurpation."
Subsequent actions 
In early May, Lincoln gave a second call for an additional 42,000 men. On May 3 President Lincoln issued a further call for United States volunteers to serve three years, unless sooner discharged. He increased the regular army by 22,714 men and called for 42,034 more volunteers to enlist for three years. In July 1861, the U.S. Congress sanctioned Lincoln's acts and authorized 500,000 additional volunteers.
- p. 1 Newell, Clayton R. & Shrader, Charles R. & Coffman, Edward M. Of Duty Well and Faithfully Done: A History of the Regular Army in the Civil War U of Nebraska Press, 01/07/2011
- p.324 General index to the laws of the United States of America from March 4, 1789, to March 3d, 1827 ...: Compiled, in pursuance of an order of the House of Representatives of the United States, of May 15, 1824 United States, Samuel Burch, United States. Congress. House, William A. Davis 1828
- The Prescott Transcript of April 20, 1861
- pp. 106–108 Lankford, Nelson D. Cry Havoc! The Crooked Road to Civil War, 1861 Penguin Books 2007
- p. 23 The American annual cyclopedia and register of important events ..., Volume 15: Embracing political, civil, military, and social affairs: public documents; biography, statistics, commerce, finance, literature, science, agriculture, and mechanical industry 1876