I was looking through the guide and saw the espionage section mentions you could delete enemy military. Is that a particular mission?
Deleting units is a bi-product of bankruptcy
When a Civilization goes below a certain gold value of a unit that unit will disband at the beginning of the next turn.
This is particularly effective right after someone upgrades their units to attempt a timing push against another player
(The target for Siphoning Gold upgrades units and you steal gold from a treasury that is at 0 while they are maintaining expensive, new units)
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Other effects are also shown there -- a loss of amenities, meaning all yields will be decreased. This compounds extremely quickly as amenities apply a -% modifier to all yields which in turn makes it harder to get gold, which makes you lose more amenities because you're unable to fund your infrastructure and armies.
In the screenshots in Espionage (which isn't filled out I just put those in so people could see the effects)
The player whose military I show as 0 had a military score well in excess of 2,500, consisting of a HUGE navy and a massive submarine force -- I was nuked around 25 times in that game. But because I was utilizing spies in the cities that I could see and properly moved them around, upgraded, planned in advance to use them he could not counter them (even as France) eventually even though he got to nukes first, I got to the required civics in advance, planned my government district around Spies, and leveled up my spies properly so that even after my capital and my first 4 (core) cities fell I won from mere attrition. he was at -18 amenities in his capital and -13 in any city over population 7 with no possibility of recuperation when the game ended