<P> In April 1914, during the Mexican Revolution, the Tampico Affair occurred, and escalated as a result of a twenty - one gun salute (or more specifically, the lack of one). Nine unarmed U.S. sailors were arrested in Tampico, Tamaulipas, Mexico, for entering an off - limit area at a fuel loading station . Despite being released, the U.S. Naval commander demanded an apology and a twenty - one gun salute . The apology was provided, but not the salute, giving President Woodrow Wilson reason to order the U.S. occupation of the port of Veracruz . Due to an arms embargo from the United States, issued to try to lessen the bloodshed of the revolution at its border, Mexico was forced to seek arms from European, and to a lesser extent Asian countries . During the occupation, a large German arms shipment aboard the SS Ypiranga was illegally seized, before being released and turned away by the U.S. occupation force . These events ultimately drove a wedge between Mexican - U.S. and German - U.S. relations, and after the start of World War I on 28 July 1914, would cause Germany to seek to influence Mexico into declaring war on the United States, in order to halt U.S. arms shipments to the Allies, and attempt to prevent the then isolationist United States from joining the Allies . This effort ultimately backfired, after culminating in the sending of the Zimmermann Telegram from Germany to Mexico, which urged Mexico to declare war on the United States, promising support and the return of much of the territory lost during the Mexican--American War, which, after being intercepted, caused the U.S. to enter the war on the side of the Allies on 6 April 1917, and due in large part to the arrival of fresh U.S. troops into the war, largely a stalemate in its first years, would lead to the surrender of Germany on 11 November 1918, after helping to overpower the German Army, already fatigued from a long and costly war . </P> <P> On Memorial Day, batteries on military installations fire a 21 - gun salute to the nation's fallen . As well, batteries at Naval stations and on ships, fire a salute of 21 - minute guns and display the ensign at half - mast from 8 a.m. until completion of the salute . </P> <P> Today, a 21 - gun salute is rendered on the arrival and departure of the President of the United States; it is fired in concordance with four ruffles and flourishes, which are immediately followed by "Hail to the Chief"--the actual gun salute begins with the first ruffle and flourish, and' run long' (i.e. the salute concludes after "Hail to the Chief" has ended). A 21 - gun salute is also rendered to former U.S. Presidents, foreign Heads of State (or members of a reigning royal family), as well as to Presidents - elect . In such a ceremony, the national anthem of the visiting dignitary's country is played, following the salute . </P> <P> In accordance with the ceremonial standard operating procedure (SOP) of the 3rd US Infantry (The Old Guard) the various gun salutes are assigned as follows: each round in a given salute is fired one at a time . The number of cannon used in a battery depends upon the intervals between each round fired . This includes, for example, a three - gun battery firing two of its guns with five - second intervals between rounds and one gun remaining at the ready in case of a misfire; such a battery would be used at an Armed Forces Full Honors Funeral, or for State Arrival Ceremony of a foreign dignitary at the Tomb of the Unknowns in Arlington National Cemetery . A four - gun battery has its first three guns firing rounds at three - second intervals, with the fourth gun (again) at the ready in case of misfire . </P>

Who is entitled to a 21 gun salute