- Apr 1 Shenandoah Valley campaign, Jackson's Battle of Woodstock, Virginia
- Apr 4 Peninsular Campaign, launched in southeastern Virginia, aimed at capturing Richmond during US Civil War begins, Confederate victory (US Civil War)
- Apr 4 Siege of Yorktown, fought in York County and Newport News, Virginia begins (Yorktown campaign), inconclusive result (US Civil War)
- Apr 5 Siege of Yorktown, fought in York County and Newport News, Virginia ends (Yorktown campaign), inconclusive result (US Civil War)
- Apr 6 Battle of Shiloh, Union defeats Confederacy near Pittsburg Landing, Tennessee
- Apr 7 Grant defeats Confederates at Battle of Shiloh, Tenn, Island #10 falls
- Apr 8 John D. Lynde patents aerosol dispenser
- Apr 11 Confederates surrender at Fort Pulaski, Georgia
- Apr 12 James Andrews steals Confederate train (General) at Kennesaw, Georgia
- Apr 12 Union troops occupy Fort Pulaski, Georgia
- Apr 14 Battle of Fort Pillow, Tennessee
Dickinson Writes to Higginson
Apr 15 American poet Emily Dickinson first corresponds with author and future literary mentor Thomas Wentworth Higginson, a relationship that lasts the rest of her life
1st Pasteurization Test
Apr 20 First pasteurization test is completed by Frenchmen Louis Pasteur and Claude Bernard
- Apr 21 Congress establishes US Mint in Denver, Colorodo
- Apr 21 Ellen Price Wood's "East Lynne" premieres in Boston
Capture of New Orleans
Apr 25 After the Battle of Forts Jackson and St. Philip, Union forces under Flag Officer David Farragut request the surrender New Orleans, LA, despite being largely defenseless armed mobs reuse
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