Weapons can leave lasting effects during and after the war. We have found certain number of banned weapons but very probably used in some terrorist organizations such as the case of Syria.
Yes, energy cannons are a real thing. Today, the Air Force uses massively powerful lasers to intercept incoming missiles. Some have been rumored to work from over 100 miles away.
Hypothetically, directed energy weapons could be mounted on tanks and pointed at humans and other fleshy targets. However, directed energy weapons with enough power to kill human targets are forbidden in war. Energy weapons are currently forbidden, in large part because too low a dose of microwave lazers at too great a distance would not kill the target so much as just melt their eyes.
This doesn’t apply to de-powered non-lethal microwave emitters used to disperse riots.
These old fashioned death traps are have only been prohibited recently in 1979. Pits with sharpened bamboo spikes maimed thousands of soldiers in Vietnam during WWII. Often these spikes were found covered in animal feces
Mustard Gas was almost single-handedly responsible for the 1928 Geneva Conventions. When inhaled, this gas causes the lungs to fill with fluid, essentially drowning victims standing.
Mustard Gas is heavier than air, and proved particularly effective in getting into the trenches of WWI, and the newspaper headlines at the time.
One of world’s oldest known arms agreements, outlaws the use of poisoned bullets. Old guns were inaccurate, so soldiers would often supplement the lack deadly firepower by soaking their bullets in poisonous or infectious substance. Ammo was often stored in latrines. Casualties often came not from gunshot wounds, but from infection.
Essentially Human bug spray. Systematically outlawed since 1899. All nerve agents work in the same basic way: By blocking enzymes and destroying neurotransmitters. Death only happens after painful blisters, boils, and internal hemorrhaging..
Interestingly, the same laws that outlaw Nerve Gas, outlaw personal defense pepper spray in times of war. It is an aerosol chemical weapon that disrupts the breathing.
Super cheap to produce. Landmines that use non-metallic fragments, and are not detectable by X-Ray are prohibited in war. Because field surgeons can’t remove fragments.
Hollow-point bullets or exploding or expanding projectiles, were labeled as “making death inevitable” for soilders, rather than just removing them from the battlefield.
Locusts, Fleas and Rats
The Black Plague is theorized by some to be the result of a bio-terror attack from Asia. Today, using hordes of animals carrying disease is illegal in conventions of wars.
Salted bombs are true nuclear weapons specifically created for the purpose of short-term land-area denial.
A “salted” nuke contains an isotope of a secondary substance such as gold, or sodium. After the blast the salt elements becomes a huge cloud of fallout. So, smaller salted nukes could be used tactically to create radioactive fallout for a year or two, keeping large swaths of land out of the enemy forces. These weapons are generally prohibited because of their potential lethality to civilians.