What have been decisive weapons?

[Knight of the Rose]

Technological advances: bronze, iron, gunpowder, the automatic machine-gun (ie gatling), aircraft armored vehicles to mention a few.

The unusual use of different weapons are another story methinks

/KotR

[spmetla]

The bombs 'little boy' and 'fat man'

[KarlXII]

Tank and aircraft.

[Husar]

http://shock.military.com/Shock/vide...soldiertech.nl

Covers only the last 60 years and has a slightly different theme but maybe you will find this of some use.

[PanzerJaeger]

The tank and the German tactics associated with it.

[Ramses II CP]

As has been suggested above, you can't isolate weapons from the tactics they required to be successful or decisive. For example you might want to say the change in the length of the Zulu spear was decisive in the rise of that people under Shaka, but without a change in the tactics with which they waged war, a sociological change in the way soldiers were recruited, and far more severe training practices the shorter spear would've been merely a footnote.

You also have to define what you want decisive to mean; decisive in one battle? In one campaign? For one generation? I think some of the most dramatic changes in warfare have come due to relatively minor alterations to current technology, much of it not even weapons. Consider the invention of stirrups, for example. Cavalry operating with stirrups would immediately have a substantial field advantage over cavalry without it... but unless the enemy were utterly eliminated they could carry that idea from the field with them and implement it themselves. The stirrup was exceptionally decisive on any number of battlefields, and perhaps even a few campaigns, but it was quickly and easily adopted by people defeated by it, so it was not overall a world changer.

I can really only think of one weapon whose use was, at least in the minds of the military leaders of the world, so decisive that it changed the nature of fighting war itself. The obvious, nuclear weapons. I think Korea, Vietnam, and Afganistan are clear demonstrations that the same essential tensions which caused the first two world wars would've caused more of the same in the absence of the fear, whether justified or not, surrounding the nuclear weapon.

I accept, however, that this may be a limitation of the fact that I'm alive right now rather than alive right after WWI when I might've thought the same of chemical weapons or machine guns.

[The Wizard]

None of those mentioned in the OP were warfare-changing weaponry and therefore don't fit the bill.

I'd say the most influential are the repeating rifle and the machine gun, which gave European (Western) armies their first and only decisive technological advantage over their non-Western rivals, and for a short time (approximately two to three decades) gave them an unparalleled ability at war. That this period coincides with the Scramble for Africa is no surprise.

A trailing third, after those two tied at first, I'd say would be the airplane. And maybe, just maybe, the tank for fourth place.

[Fragony]

None of those mentioned in the OP were warfare-changing weaponry and therefore don't fit the bill.

In the case of the longbow and goedendag, ok, but the bayonette? You kidding me it was a revolution in warfare, it make a charge dangerous because of the firepower, and it made shields useless because of the side-stab (attacking the guy the guy next to you is fighting) and has stayed the dominant weapon on the battlefield for decades to come and it's still used even today, probably the most revolutionary weapon out of all of them.

[Mangudai]

The tank and the German tactics associated with it.

Yeah... I think the German victory over France was half genius and half luck. The French (and BEF) played perfectly into German hands. The allies could have stopped the blitz if they deployed more intelligently.

[Sabuti]

The first club! Me smash head with stick, me win.