What if Teddy Roosevelt was killed at the Battle of San Juan Hill? How would this affect the war in Cuba? What are its effects in the US in the coming years? Who's going to succeed McKinley if he's still assassinated?
(1) TR's death is not going to make any difference in the outcome of the Spanish-American War. "The ensuing war was pathetically one-sided, since Spain had readied neither its army nor its navy for a distant war with the formidable power of the United States."
http://www.britannica.com/EBchecked/topic/558008/Spanish-American-War
(2) As for McKinley's running mate in the event of TR's unavailability, I'll quote a post of mine from soc.history.what-if:
***
We'll assume that Theodore Roosevelt is killed in the Spanish-American War
or loses the 1898 race for Governor of New York, or that leading
Republicans outside New York object to Boss Platt's attempt to remove him
from New York by giving him the vice-presidential nomination (in OTL Mark
Hanna rebuked them in vain, reportedly asking them "Don't any of you
realize that there's only one life between this madman and the White
House?") or that McKinley responds positively to the appeal of Senator
Charles Dick, Secretary of the Republican National Committee, to stop the
TR boom. [1] (Another possibility, of course, is TR himself declining, as
he said he would do--but the pull of party loyalty and the need to do
whatever was necessary to defeat the unspeakable Bryan outweighed his
reluctance to give up the Governorship of New York for the vice-
presidency. TR had written to Henry Cabot Lodge in 1899 that "The Vice-
Presidency is a most honorable office, but for a young man there is not
much to do. It is infinitely better than many other positions, but it
hardly seems to me as good as being Governor of this State, which is a
pretty important State...If I am Vice-President I am 'planted' for four
years. Here I can turn around.")
So who are the alternatives to TR, one of whom will become President in
1901 unless the assassination of McKinley is somehow butterflied away?
(1) Elihu Root, Secretary of War, might have had the job if he wanted it,
but he announced several months prior to the Convention that he would not
seek it. (It was soon after Root's statement that the TR boom gathered
strength).
(2) John D. Long of Massachusetts, Secretary of the Navy since 1897 (he
had been Governor of Massachusetts and then a Congressman from that
state). According to
http://en.wikipedia.org/wiki/John_Davis_Long he
"served with vision and efficiency through the next five years, organizing
the Navy for the challenges of the Spanish-American War and the expansion
that followed, and laying the groundwork for the growth of the 'New
American Navy' fostered by his former assistant, President Theodore
Roosevelt." So if he became President he could be much like TR in terms
of advocating an expanded navy and a vigorous foreign policy; I am less
certain, however, about his domestic policy.
(3) Cornelius N. Bliss, ex-Secretary of the Interior under McKinley. He
declined to seek the vice-presidency in OTL.
http://en.wikipedia.org/wiki/Cornelius_Newton_Bliss "A consistent advocate
of the protective tariff, he was one of the organizers and for many years
president of the American Protective Tariff League." Well, we can be sure
he is not going to do anything to lower the tariff--though come to think
of it, TR didn't do much in that direction, either. But even apart from
this, he seems likely to be considerably more conservative than TR.
(4) Senator William B. Allison of Iowa.
http://en.wikipedia.org/wiki/William_B._Allison Kevin Philips has
suggested that in 1896 he was the only possible GOP presidential candidate
other than McKinley who could have defeated Bryan: "Among those favored
by the machine chiefs, the most plausible winner was Iowa Senator William
B. Allison. Respected in the Senate, he had coauthored major silver
legislation in 1878 with Missouri Democratic Congress­man 'Silver Dick'
Bland. Despite his age (sixty-seven) and lackluster public persona, he
might have been able to hold most of the Midwest. If so, Quay, Platt, and
the other Eastern leaders presumably could have carried their own
bailiwicks for Allison against a Bryan caricatured as a lineal descendant
of Marat and Robespierre."
http://groups.google.com/group/soc.history.what-if/msg/7cbff16a87af9190
But Allison's age (71 in 1900) does present a problem for the vice-
presidential nomination and makes it questionable whether, should he
become President on McKinley's assassination, he will seek a full term in
1904. (He might; after all, in OTL he served in the Senate until his
death in 1908.)
(5) The other Senator from Iowa, Jonathan P. Dolliver.
http://en.wikipedia.org/wiki/Jonathan_P._Dolliver Born in 1858, he was
almost as young as TR. As noted in the 1910 New York Times obituary at
http://query.nytimes.com/mem/archive-free/pdf?res=9900E4DA1F39E333A25755C1A9669D946196D6CF
he became an "insurgent," an ally of Senator La Follette, and a critic of the
Payne-Aldrich Tariff (though he had previously been a strong protectionist)
toward the end of his life. As of 1900 he had been considered a "stand
patter"--a typical McKinley Republican (he delivered a widely noted speech in
defense of the annexation of the Philippines) and an ally of Allison. After
McKinley's death he became a staunch supporter of TR.
(6) Charles W. Fairbanks of Indiana. I discussed him at
http://groups.google.com/group/soc.history.what-if/msg/6c54c8e9aee6b351
He is said to be McKinley's own choice as a successor. But precisely for
that reason, I doubt that McKinley would want him as a running mate--in
those days, the Vice-President was rarely a serious contender for the
presidential nomination. But McKinley seems to have sincerely believed
that choosing the vice-presidential nominee was the business of the
convention, not of the president or presidential nominee (this is one
reason why he made no attempt to block TR, though he was not very
enthusiastic about him).
Most of the alternative candidates look like they would be more
conservative than TR--Dolliver's move to progressivism was made later and
it is questionable whether it would have happened without the example of
TR as President. (Indeed, some of them might have been more conservative
than McKinley himself, who sought good relations with organized labor and
who in his last speech showed a willingness to modify his protectionism
with reciprocity treaties. It is even possible that McKinley would have
initiated the Northern Securities antirust case himself; Hanna was later
to say, "I warned Hill that McKinley might have to act against his damn
company last year. Mr. Roosevelt's done it. I'm sorry for Hill, but just
what do you gentlemen think I can do?") OTOH, I think that at least some
of the alternative candidates would have looked more closely at the option
of turning to Nicaragua after the Colombians turned down the proposed
canal treaty, instead of encouraging revolution in Panama. This might
have slightly delayed the building of a canal, but would have left Latin
Americans less angry with the US.
Of course some of these candidates, especially the older ones like
Allison, might not have run for a full presidential term in 1904. In that
event, possible Republican candidates that year might include TR himself
(if he had managed to be re-elected Governor of New York--but conservative
opposition at the national convention would be hard to overcome) or Hanna
(if these events butterfly away his fatal typhoid fever of early 1904) or
Root or Lodge or Joseph Foraker, whom I discuss at
http://groups.google.com/group/soc.history.what-if/msg/a590c23e8a78169e
[1] Though even if he did so, it is not clear that he would be successful,
given that TR had considerable support in the West. Charles G. Dawes did
not want TR selected, but he warned "There is an inclination now for the
administration to step in along later in the night and announce that
perhaps it is best for Mr. Long to be the candidate. I think that is
based on a wrong diagnosis of the situation. I think as soon as that is
announced that the delegations of the west will say,--'That's dictation'--
and will change their votes to Roosevelt..." Walter LaFeber, "Election of
1900," in Arthur M. Schlesinger, Jr., Fred L. Israel, and William P.
Hanson (eds) *History of American Presidential Elections 1789-1968, Volume
III*, p. 1937.
https://groups.google.com/d/msg/soc.history.what-if/mMhFKXMfSVU/-411NnI85KgJ