Warfare has always gone beyond the simplicities of soldiers fighting soldiers. Both sides of the battle have always scrambled to come up with some new angle, to find a new way of fighting that would give them the advantage and let them win the day. It wasn't uncommon for medieval armies to attempt breaking sieges by catapulting plagued corpses over city walls, and if the Romans had been able to salt the fields of Carthage without first grinding that city to powder, they may well have taken the opportunity to push their Carthaginian rivals even further back against the wall. Under Operation Popeye, which ran from 1967 to 1972, the United States military seeded clouds over the Ho Chi Minh Trail with silver iodide to induce them to give up their rain and thus extend the normal monsoon season.
This kind of warfare is nothing new, even though it's only been within the last hundred years that armies have gained the ability to even begin to use the weather as a weapon. Fortunately, it didn't have much of a chance to get established - the Environmental Modification Convention, banning the use of weather modification-based warfare, has been ratified by pretty much every major player save France. The United States, Russia, China, and the United Kingdom are all on board. Nevertheless, I think that there's a fair chance environmental warfare may come to be used in the twenty-first century.
If you only pay attention to North American news sources, you may have missed the story of Shen Neng 1, a Chinese coal ship that ran aground in Australia's Great Barrier Reef on Saturday. The Australian government is understandably furious - beyond the potential damage to the reef if the ship breaks up and dumps its fuel oil into the ocean, the area where it ran aground is within a nautical exclusion zone. Australian Prime Minister Rudd has not been sparing the tongue-lashings, and it's probable that a hefty fine will come out of this.
I know that one should never attribute to malice that which can be adequately explained by stupidity. A lot of people know this. More than enough to take advantage of it. As the twenty-first century unfolds, I wouldn't be surprised to see a rise in instances of environmental warfare - by which I mean deliberate, destructive actions targeted at the environment, specifically a particular state's environment. Proxy war without bloodshed - just forcing a state to spend resources and effort remediating an environmental disaster. It wouldn't have to be something as blunt as an oil spill in a sensitive reef, either - invasive species can cause billions in damage over a very short time, and devastate carefully balanced ecosystems. Witness the effect of pine beetles in the northern forests of British Columbia and Alberta, the scourge of zebra mussels in the Great Lakes, or the infestations of Asian long-horned beetles across North America.
The fact is, too many world leaders don't appreciate the reality of Spaceship Earth, and that there's no form of environmental warfare that wouldn't come back to bite the agency that began fighting with it. I don't think that the Shen Neng 1 incident was some kind of Chinese black op. But I wouldn't be surprised to see something like it repeated in the future.