For years, Athens had two dominant men, the conservative Nicias and the flamboyant Alcibiades who countered each other’s designs at every turn.
Nicias wanted to mostly focus on defending Athenian territories and ending war with the Spartan alliance if possible.
Alcibiades wanted to take the fight to the Spartans.
Had either of these men gotten their way, it seems likely that Athens would have survived.
The combination of both fighting for power at every turn destroyed Athens.
When Nicias succeeded in making peace with the Spartans, Alcibiades sabotaged the peace.
When Alcibiades wanted to fight in Sicily, Nicias tried to sabotage him.
The ultimate worst case scenario of democracy followed:
Nicias made a public speech in which he proposed an absurd force both in size and composition that would be required to invade Sicily.
His intent was to discourage the public from supporting the expedition.
Instead: the masses loved his idea. They loved it so much that his absurd proposition became the actual Athenian force.
Nicias, who wanted to halt the entire operation was elected to be one of the generals commanding the expedition against his will.
Things could still have turned out alright.
But even as the invasion fleet was headed for Sicily, Alcibiades was recalled to Athens for accusations of treason and sacrilege.
They were still arguing over who was in command and sabotaging one another while on the way to battle!
Alcibiades had to go back home to stand trial: he was the guy who had wanted a Sicilian Expedition in the first place!
And Nicias was now commanding an expedition he had actively opposed!!
Knowing he would likely face death and disgrace back at home, Alcibiades escaped and actually defected to the Spartans.
Now not only was the huge Athenian invasion force already a confused mess, Alcibiades was in Sparta giving the Spartans valuable military intelligence and advising them to send their own expedition to counter the Athenians in Sicily.
Even now things might not have been a total disaster but bad decisions continued to combine to create the perfect storm.
Not only did the Athenian invasion force fail: it was wiped out.
And so an enormous amount of wealth and a large portion of the entire male population of Athens had just been flushed down the toilet.
Athens was doomed.
Also most of what I’ve written here I got from reading Donald Kagan’s Peloponnesian War.