This is an addition to the GURPS Infinity World Reality Mather: the "Withering War" generally refers to the decades-long conflict that took place on that timeline between humanity and organized monsters.
The Withering War in the United States
The basic strategy of the US was always clear-cut; it was the implementation that was the trouble. To win, the United States needed to: first, hold some sort of defensive line while the country could fully mobilize; second, eliminate all infection inside the line; and third, aggressively smash up all forces outside of the line, defeating each group in detail before they could unite on their own. A well-equipped and armed regiment could easily destroy a demon, lich, monster pack or master vampire, as long as the regiment could get within rifle range, and there were more regiments than there were monsters. The difficulty, of course, was making sure that the regiments were able to get to where they were needed.
American strategy thus relied heavily on transportation, particularly railroads and riverboats. The former dictated that the defensive line would be centered around the Northeastern United States (which was also the area of heaviest industry), while the latter kept the Midwest and South from being totally isolated and overrun. The Great Plains and Rocky Mountains were abandoned in favor of keeping access to California; the garrisoning of the land route (and first transcontinental rail line, beginning in 1865) also kept the southern border under control.
By 1862 the United States had managed to stabilize the Northeast from New Hampshire to Ohio in the west, and Virginia in the south. The rest of the Atlantic and Gulf coastal regions were more or less under human control, although many regions were effectively cut off by land. In the Midwest, the center of human strength was Illinois and parts of Missouri, which were also cut off from the government in Washington.
The first set of campaigns was designed to reestablish full control over the Appalachians, as a prelude to securing the Mississippi River. This was accomplished by the middle of 1863... at which point the first serious political controversy arose. Up until this point, American armies typically consisted of Northern soldiers led - usually ably - by Southern officers. Generals such as Lee, Jackson and Thomas were instrumental in clearing out the eastern United States of major infestations, although smaller ones persisted for some time.
However, the fighting in the Midwest and Pacific coast had produced winning generals such as Grant and Crook in the former and Sherman and Sheridan in the latter. These areas tended also towards support of the Republican Party, providing a political dimension to the war that had previously been muted. There was a widespread belief in the Midwest - and to a lesser extent, the South - that the Northeast had been slow in helping the rest of the country. There was also some speculation over whether the 1864 elections might be suspended as part of the emergency. While neither rumor appeared to be remotely true, the Buchanan administration judged it necessary to show favor to Midwestern and Western armies and their generals.
This proved to be awkward, given that starting in 1860 the Army of the Illinois had been enlisting large numbers of displaced black refugees from the lower Mississippi. The combination of limited resources and relatively low population dictated that the army would take anyone willing and able to shoot and march; the pay was noticeably less for black troops, but was also largely theoretical for anyone at this point anyway. By the time that Federal control was re-established over Illinois a good number of these refugees - which undoubtedly included former slaves - were advancing in the non-commissioned ranks; there was even a bare handful of black officers, one of whom was actually commanding white troops. The first attempts to change this by the Department of War were sufficiently ham-handed as to nearly start a civil war.
The matter was eventually settled by the Manumission Decision of 1864. Spearheaded by Senator Abraham Lincoln of Illinois, it offered freeman status to any slave volunteering to serve in the United States Army for a period of five years. The government agreed to pay fair compensation to the owner of the slave in exchange for said service; and the children of freedmen automatically shared his or her status (these latter points became moot once the Fifteenth and Sixteenth Amendments were passed in 1871). It was also agreed that freemen would not be even offered the right of franchise until at least 1872 (this latter added to reassure Democrats worried about new Republican voters).
There was surprisingly little controversy over this decision: seven years of war had devastated the plantation system upon which American chattel slavery was based. The promise of payment turned out to defuse most objections. For that matter, the East and South were beginning to strain to keep their troop strength up. Generals Lee and Forrest were the first commanders to accept large numbers of freedman enlistees; while no wartime force under their direct command ever promoted any black beyond the rank of corporal, the latter did preside over the full integration of the American military in 1874, on direct orders from the former as Commander in Chief.
What complaints arose came from South Carolina, leading to the infamous Interdict of 1864. Negotiations between the state government and President Buchanan had rapidly declined to the point where South Carolina was openly threatening secession over the proposed bill of manumission. The Buchanan administration chose to handle the matter by isolating the state as much as possible; the task to do so was given to General Sherman (at the time on convalescent leave), and he did so with such rigor that his name is regularly cursed by the survivors, thirty years later. It is an exaggeration to claim that the Interdict killed every person in South Carolina; the casualty rate barely reached decimation levels, and that only because of an unforeseen zombie attack in Columbus itself. But the United States government clearly decided to break one state to demonstrate its power to the others, and the other states took the hint.
Clearing the Gulf Coast to acceptably safe levels took another three years (although the most inaccessible portions of Florida and Louisiana are risky, even in 1892); the Mississippi was officially reclaimed by 1869. The next sets of campaigns (1870 and 1872) took back control of the Missouri River; from that point it was a matter of the Rocky Mountains and the Deseret relief campaign... which was easily the bloodiest period in a war that killed over half a million people.
Aftermath of the Withering War
Like Homeline's Civil War, the Withering War in the United States established once and for all that the country was a Federal Republic, and not a voluntary compact of semi-independent States. Unlike the Civil War, this was accomplished without significant long-term bitterness; it became clear very quickly that only a unified country was going to survive the invasions. While the damage done to the USA's infrastructure was about equal in both instances, it was spread out further in Mather, and more quickly repaired. The South's total industrial capacity in 1892 is just short of that of the North's, and its rail network might actually be slightly more extensive.
The post-Withering War period has seen a decided expansionism: the collapse of Mexico encouraged American intervention there even before the final campaigns to relieve Deseret. At the present time, the USA has occupied Mexico down to about Durango: while the rest of the country is technically part of the Republic of Grand Colombia, the latter is simply currently incapable of maintaining order north of Puebla. For that matter, Grand Colombia has no particular desire to get into a war with the United States. The two countries are currently discussing old plans to build a canal across the isthmus, which would be much more valuable to the Colombians than a perennially-chaotic, restive territory. Meanwhile, in the north the United States quietly annexed the entire Oregon country in 1873, over the largely-symbolic objections of the British Empire. The reasons given were a mix of practicality and humanitarianism (the British were finding it impossible to keep the Pacific Northwest territories supplied) - or, as the American ambassador to England at the time put it, "If Queen Victoria didn't want to use British Columbia, President Lee would like to borrow it for a while." Further expansion into Canada is generally frowned upon, but the possibilities of former Russian Alaska appeal; there is already a considerable freelance American missionary/settlement program in place.
While the USA is a much more egalitarian place after than its 1890s Homeline equivalent, there are still lingering prejudices. Blacks have probably the easiest time of it, given their continuing presence in the American military: the only real taboo left for them is intermarriage with Whites. Mexicans from the conquered parts of Mexico and "civilized" Indians (such as the ones in Oklahoma) suffer from Social Stigma (Second-Class Citizen); Indians from the Plains are lucky to be treated as Uneducated (the more recalcitrant are treated as being part of a Minority Group). Most Orientals are considered part of a Minority Group, with additional penalties for being Ignorant of Western social mores. On the other hand, religious intolerance is almost nonexistent, and the suffragette movement is considerably further advanced in this universe than it was at the equivalent point on Homeline.