The Ninja was a scout, infiltrator, and assassin that disguised as enemy soldiers or common civilians to perform missions that brutish warriors and their massive armies could not do. Their warfare was unconventional, small-scale, secretive and surprising.
While there are no more Ninjatos or ‘magical’ Jutsus; the concept of a ‘Spy’ still exists today and has modernized appropriately. James Bond, TF2, Splinter Cell, Metal Gear, Total Spies, Kim Possible, Black Widow and even Batman are great examples of Spy combatants; except that they are fictional. Sadly while Spy-Watches do exist; they don’t have Lasers and Invisibility Cloaks (unless the CIA has censored that information so well that I’m just oblivious).
Sabotage, hacking, traps, poisons all examples of how modern Spies do NOT go into a mush-pit like Batman to kung-fu 50 guys at once, but instead avoid direct combat entirely. The skills of a Spy are more about maintain stealth and being capable of committing to the mission: not combat. If you are in the heart of enemy territory, then the entire army and national police will go after you once they identify the Spy as a spy. But as already stated; just pretend to be them or bribe enemy units to your side as double-agents. The USA’s CIA has quite the advantage with the wealth to bribe entire nations with ease and the ethnic diversity to find a native of the enemy nation. Although this goes both ways; your Panda Express chef might be a Chinese communist, the actors of your local Little Mermaid play might be CRAB PEOPLE.
Let’s consider a battle between a James-Bond-esque squad of spies fighting a squad of Green Berets, and let’s assume the Spies are acting like regular soldiers: both sides have their identities clearly confirmed, both have no body armor, and both are engaged in the front lines. The Spy is dressed in civilian clothing, probably a suit, which is clearly not camo: Berets do wear camo. Automatically, the Spy is an easier target. Berets clearly have rifles, and most squads have at least one sniper: Spies use concealed weapons; assuming they don’t have them hidden in rose-boxes (a la Terminator 2) then the best ranged weapon is some pistol. For anyone who plays TF2; try fighting as a TF2-Spy on a 100% Crit server, without using your cloak, your pistol has a realistic 50 yard effective range, the map is several miles long, and the enemy team has capable Snipers: you have a better chance winning the lottery.
MAD’s Spy vs. Spy comics are surprisingly more realistic in depicting actual Spy combat: at least compared to James Bond. Assassination is the focus; achieve the ambush, score the kill, get out smooth and fast. This is exactly NOT what Deadliest Warrior depicted in their CIA/KGB episode; the moment that a public killing occurs, the Spies are identified, and the fight dissolves into a loud and chaotic gunfight. If this (a gunfight between two squads of Spies occurring in public) were to happen in real life; it could potentially have been classified as an Act-of-War and any surviving Spy would be immediately arrested: because calling the police is what you do if you hear a gunfight near a government building.
Assassinations are warcrimes more or less, so don’t try this in real life kiddies. So let’s assume that we are trying to assassinate Hitler; you do NOT suspiciously sneak up behind him and garrote him. Hitler has bodyguards; you need to approach him at point-blank range and use a weapon with a very specific and slow killing method, this is never going to work. I have 3 plans; public, planting and poisoning.
Hitler loves speeches, he loves attention; killing him at a speech is easy to setup. The hard part is doing this while ALSO being able to get away. Sneaking in a sniping-rifle might not work; the Twilight Zone episode “No Time Like the Past” shows how easy it is to get caught or miss, even if you make it to the sniping post. Solution: be a cameraman, use a Camera-Gun. Metal detectors (if they existed at this time) determine your Camera to be safe. Hitler poses in front of literally dozens of cameramen; the Spy included. Easy target: BANG! Chaos erupts, no one can find the shooter, the crowd stampedes, and the Spy hides in the crowd. Worst case scenario; the Spy is caught, but uses a cyanide-capsule in the Camera to keep his secret: you lose one spy, but the world loses one genocidal war monger.
Planting a bomb; Tom Cruise’s Valkyrie is all about that ‘What if’ moment where Hitler was almost killed by a bomb. If the bomb was more powerful, it would have worked. Carbombs are frequently used since they look like any other car; it’s easy to drive it anywhere in a city. Maybe tripwire something Hitler personally owns; Sniper Elite game did this with a soup spoon of all things, and the killing of Yahaya Ayyash was done by putting a bomb in his cellphone.
Poisoning is strait forward: and is a lot easier to do than you would think. Tasteless, incurable and lethal poisons are frequently used by Spies; Alexander Litvinenko was assassinated with Polonium-210. This isn’t to be confused with Plutonium, but it’s still just as deadly. I guess if you want to guarantee a kill, you use nuclear elements.
So as mentioned before with Ninjas; the Spy is capable of self-defense, but wants to avoid that. They are to assist the warriors, not to do the fighting.