My interpretations on above poetry:
Dihydrogen Monoxide
Spoiler:
Structural breakdown:
Dihydrogen Monoxide
To begin, I wanted a name that works to its favor in such a way that something as simply as water could be considered a vicious being. This ties in with a hoax not too long ago using the exact same name (which is the chemical formula for water). Perhaps something as innocuous as water would be a major turning point in life.

Written upon the epitaph of life
It is true that we need water to live. It is also true that we can die from water either from drowning or even excessive water intake. The dynamics of water comes from the idea that it is central to our life as we know it. We cannot separate life or death from water and we certainly cannot separate our actions as people and water usage.

Fallen, a flagbearer through strife
Fallen: rain
The idea that it is a flagbearer through strife is a duo dynamic. The first is if we understand the situation of droughts. Obviously rain is incredibly good for crops and agriculture in general. So the idea that it is a flagbearer through strife shows that it is one who brings hope to people. This is why traditions have rain-dancing and rainmakers. It is key to survival and without such rain there is starvation and strife. On the other hand, it can be said that rain kills. I need not point to floods in Queensland to show that rain in itself is also the flagbearer of strife to people. This dynamic is important as we should view water as both a mechanism of life, but it is in itself a double-edged sword.

Such a trebuchet, foreshadowing both loss and victory
A trebuchet is a weapon yes. It lays siege upon castles in the past. Why would one describe water as a trebuchet? The idea that trebuchets are a key component of any army perhaps is not about water being a weapon (but it certainly can be), but that it is a key component to why we fight. Thus it foreshadows battles in the future. Our future that may somehow be fought over water.

The irony of this statement is that wars need not be with bloodshed and firearms either. Something as simple as controlling water supplies to a country would have huge impacts on their livelihood. Thus economic wars can use water in itself as a siege weapon. Economically speaking battles could be won with just water.

Echoing such origins as benedictory
An interesting note to give here is that water is considered almost universally a good thing. Thus justification for water is just as much like this.

A quadriga for Mars, the elixir of Neptune
A closing statement. There is therefore a juxtaposition of two very different things. A quadriga is a chariot used by Mars to fight wars. An elixir is one that provides life, from Neptune. Water is both. How we use this is for us to ponder on. How we should think as people would therefore be simply understanding how we can live and how we deal with water as a problem in the future.