Unrestricted Submarine Warfare (URSW) is a Naval doctrine in which a submarine will attack any vessel carrying a flag of its enemies, its enemies' allies, or others suspected of giving aid to an enemy, without warning or provocation. The doctrine applies to any vessel, whether it is civilian or military in nature, large or small.

HISTORY

Prior to WWI, belligerents observed "Prize" or "Cruiser" rules, which stated that the ship couldn't sink a passenger ship, only a merchant vessel of an enemy nation, and that the crew/passengers aboard any vessel must be clear and safe before it was sunk. These rules were from the days of sailing vessels, and with WWI and the advent of submarine warfare and modern weapons, it didn't take long to determine that those rules were obsolete. This was essentially the basis for Germany's initial submarine warfare policy.

Though Germany initially tried to comply with Prize Rules, as WWI submarine warfare progressed, it became apparent that those rules were outdated and even dangerous. With its practice of warning ships and allowing passengers and crew to leave prior to sinking, it meant that the attacking submarine would be a sitting duck to any enemy vessels or aircraft. The ship's crew only needed to summon military assistance by radio, and the passengers and crew could take their time to evacuate the ship, giving time for aid to arrive.

URSW & THE SINKING OF THE BRITISH PASSENGER LINER RMS LUSITANIA

On May 1, 1915, the British passenger liner RMS Lusitania departed New York for Britain. Just a few days earlier, Germany had released this notice via its embassy in Washington, D.C.:

No, not until the attack at Pearl Harbor (1941), did America provide finances and munitions to the Allies fighting Germany and Italy.

Here is a list of those weapons and military items which were invented either during the course of the war (1914-1918), or which were invented shortly before (roughly 1900-1914) and saw their first military action during WW1:

• Browning Automatic Rifle (BAR)
• ASDIC & Hydrophones
• "Hedgehog" anti-submarine weapon
• Depth charges
• Poison gases - the first military use of Chlorine, Phosgene, Cyanide, and Mustard gas
• Aircraft - very minor experiments in aerial bombing had occurred prior to WW1, but WW1 was the "coming out" party for air power, even if the overall impact of airplanes on the war was minor.
• Armored Tanks
• Portable Flamethrowers
• "Trench" mortars
• Motorized transport - while not technically a weapon, WW1 saw the introduction of gas- and diesel-powered tractors and trucks, which replaced a good deal of the logistical transport that horses and mules had provided for millenia. It revolutionized logistics in the same manner that railroads had.
• Lightweight machine guns - guns which could be used on the move, rather than require a fixed firing position - for instance, the Lewis gun. This is the beginning of the squad fire-support role, rather than company-level fire support as provided by heavy machine guns.

These are a list of weapons which, while neither invented during or first used in a military setting during WW1, nonetheless had their first significant use during the war - that is, the real effectiveness of the weapon was first demonstrated to a large audience of military

• Submarines - while submarines had been invented in the mid-1800s, they were not technologically advanced enough to be usable as a weapon of war until the maturation of the Whitehead torpedo and reliable diesel engines, which mean that the first militarily-useful submarine was not build until close to 1910.
• Hand grenades - small hand-thrown "bombs" date to antiquity, but the modern fragmentation grenade saw its large-scale debut in WW1.
• Semi-automatic pistols - WW1 was the first time that the large-scale use of semi-auto pistols by militaries; however, trench warfare's hideously dirty environment made for a relatively poor debut, with revolvers remaining the favorite sidearm.
• Modern steel helmet - while not technically a weapon, and while helmets were thousands of years old in design, what we now recognize as an essential part of a soldier's gear was re-introduced with a whole new design.
• Amphibious infantry assaults - WW1 saw the first attempts at very large-scale combined-arms assaults from the sea across a beach.
• "Special Forces" teams - while militaries always had had elite units, and sometimes picked small groups for special assaults, WW1 saw the first organizational creation of small assault groups which used tactics, weapons, and equipment different than the traditional infantry. That is, these units were distinguishable from the ordinary infantry. The German strosstruppen were leaders in this new field.

Finally, here is a list of weapons which, while commonly associated with use in WW1, were nonetheless widely used before the war, and the military strategists either knew or should have known of their impact:

• machine guns - the Maxim machine gun (the first gas-recoil automatic gun) was designed in the 1880s, and was deployed by all armies well before WW1. It was notably used in the British Colonial Army during various colonial wars in Africa. Sadly, military circles severely underestimated the defensive power of this weapon when used in Europe.
• Whitehead Torpedo - the first practical modern torpedo was demonstrated in 1870.
• Wireless Radio - radio was a late 1800s-invention, but was only beginning to be deployed to naval vessels before WW1, and was not fully utilized well by land-forces (mostly due to the large physical size and power requirements of radios at the time).
• Efficient Diesel Engine - particularly in submarines and small naval vessels, compact, powerful, and long-range diesel engines changed the balance of naval architecture.
• Large-scale Siege Artillery - the use of massive-size artillery ( 300mm+ bores) firing indirectly was well known.
• Artillery barrages - the scale to which massive numbers of artillery would fire continuously upon an opponent's fixed position was wholly new to WW1, where the volume and scale of such bombardments dwarfed anything prior.
• Barbed wire - while barbed wire had been used in several smaller wars before, the extensive use of wire to restrict large-scale unit movement was entirely new to WW1
• Shotguns - these had been used since the early 1800s, and were in declining use in militaries by WW1. However, trench warfare brought back their popularity (particularly by the Americans).

A British ship carrying hundreds of American passengers and weapons that was torpedoed by a German U-Boat. This helped America enter the war as the population began to view Germany as an enemy.

On further review the Lusitania was carrying war weapons to Great Britain.