In our timeline, the Cunctator, having provoked the Second Punic War through diplomatic maneuverings in his old age, held Hannibal at bay in Italy for over a decade while weakening him, until Scipio forced Hannibal to return to Africa, where the Romans defeated him.
But, in another timeline, a mosquito stung the Cunctator shortly after war broke out, giving him malaria, which was then endemic in Italy. He would have recovered if not for another piece of bad luck: clumsy from the fever, he stumbled on the way to the latrine and cracked his skull on a rock, dying instantly. The Cunctator's friend and rival Gaius Flaminius was given command of the Roman forces, who attempted direct confrontation with Hannibal's forces, suffering a series of increasingly disastrous defeats until finally Hannibal marched his elephants into Rome and put the Roman Senate to the sword.
The same mosquito hatched in our timeline, but mosquitoes are not strong fliers, and the air currents were slightly different in our timeline, so it instead stung the Cunctator's slave, who got malaria but survived. Air currents are of course chaotic¹, and the divergence between the timelines has been traced to the thermal emission of a single photon from a warm rock thirteen years earlier in Karnataka, resulting in the rock being slightly cooler and producing an almost undetectably smaller thermal updraft that night.
How our universe could have ended up two-dimensional is a much more difficult question.
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¹ https://npg.copernicus.org/articles/25/387/2018/npg-25-387-2... estimates the maximum Lyapunov exponents of well-regarded atmospheric models such as PUMA in the neighborhood of 0.02, i.e., a Lyapunov time of a few months. As I understand it, that means that the 10⁻²⁰ joules of an infrared photon emission creates atmospheric disturbances of about a joule in about six years and about 10²⁰ joules in about 13 years, which is a couple of milliseconds of solar irradiance.
