20 April 1775- Immediately after the battle of Lexington and Concord, Colonial Militiamen would secure and hold all land access and routes to the city of Boston. This would lead to the start of the Siege of Boston, held by British troops, and the opening phases of the Revolutionary War. At this time the town was primarily just a peninsular city on Massachusetts Bay, with limited access to land routes. This would limit the British resupply to only what could come from the sea. Resupply issues were not limited to the British as Colonial Militia, turned Continental Army, had their own issues with personal and supplies. After initial actions, such as battles at Bunker and Breeds Hill, the opposing forces set in and military actions would be limited to the occasional raid, minor skirmishes and sniper fire. In November 1775 General Washington would send in Henry Knox with heavy artillery. Over the next several months Knox would turn the city indefensible by the British, leading to their withdraw on 17 March 1776.
20 April 1861- Colonel Robert E Lee resigns his commission from the United States Army. This happened three days after his home state of Virginia seceded from the Union and two days after he had been offered command of the Union Army. While he opposed the secession of southern states against the Union, he was a loyal son of Virginia. He would soon join the new Confederacy and take command of the Virginia State forces, being given the sword of George Washington as a symbol of his appointment.
25 April 1960- The end of Operation Sandblast. This was the code name given to the first submerged circumnavigation of the world, accomplished by the nuclear submarine U.S.S Triton. The operation would begin on 24 February. Over the next 60 days and 21 hours the U.S.S Triton would cover 26,723 nautical miles, generally following the route of Portuguese explorer Ferdinand Magellan. The operation gathered extensive oceanographic, hydrographic, gravimetric, geophysical, and psychological data while highlighting US long range submerged capabilities while remaining undetected by hostile forces.