I don't know about "modern world and modern history" part of it, but Haber's work was important for what followed in at least four major ways that I can think of:

• the Haber-Bosch process allows for mass production of fertilizers which dramatically increased the availability of food, and thus the size of sustainable populations, as noted;

• the same process also could be used for the development of explosives and armaments; without the process, Germany would not have been able to start two world wars, because their nitrate supply was dramatically dependent on foreign sources prior to this process;

• Haber himself also famously was key to the inauguration of the use of chemical weapons in WWI (he personally led a gas attack at Ypres). Many of his other chemical creations — notably the insecticide Zyklon-B — would go on to horrific use in later decades;

• Haber was something of a "model" for what could be achieved if science, industry, and the state had closer connections. Haber was an academic who worked at a state-run institute (the KWI) with close connections to industry (BASF) and tight connections to the military. Such a thing was uncommon in the 19th and early 20th centuries. After WWI, people who cared about such matters, including people who idolized Haber (like Harvard's James Conant) organized to replicate such connections in other nations (notably the USA). Haber's work is a precursor for the military-industrial complex and much else.

Not sure if that gets at what you want, but Haber is one of most important scientists of the early 20th century in my view, for the reasons outlined above. His work had huge resonances through the rest of the century, sometimes quite explicitly invoked (Conant in particular saw the "German model," and specifically Haber, as the model for what American science needed to copy).

(On Conant and Haber, see Hershberg's From Harvard to Hiroshima. On Haber himself, I am terribly fond of Steven Shapin's LRB review of a Haber biography.)