The Federalist Papers, which were circulated while the debate over ratifying the constitution was underway in an effort to argue for its ratification, are one the main sources of legislative history for the 1789 Constitution of the United States of America.
The Guarantee Clause is discussed at length in Federalist Paper No. 43 written by Madison. It says in the pertinent part:
"To guarantee to every State in the Union a republican form of
government; to protect each of them against invasion; and on
application of the legislature, or of the executive (when the
legislature cannot be convened), against domestic violence.
''In a confederacy founded on republican principles, and composed of
republican members, the superintending government ought clearly to
possess authority to defend the system against aristocratic or
monarchial innovations. The more intimate the nature of such a union
may be, the greater interest have the members in the political
institutions of each other; and the greater right to insist that the
forms of government under which the compact was entered into should be
SUBSTANTIALLY maintained. But a right implies a remedy; and where else
could the remedy be deposited, than where it is deposited by the
Constitution? Governments of dissimilar principles and forms have been
found less adapted to a federal coalition of any sort, than those of a
kindred nature. "As the confederate republic of Germany,'' says
Montesquieu, "consists of free cities and petty states, subject to
different princes, experience shows us that it is more imperfect than
that of Holland and Switzerland. '' "Greece was undone,'' he adds, "as
soon as the king of Macedon obtained a seat among the Amphictyons.''
In the latter case, no doubt, the disproportionate force, as well as
the monarchical form, of the new confederate, had its share of
influence on the events. It may possibly be asked, what need there
could be of such a precaution, and whether it may not become a pretext
for alterations in the State governments, without the concurrence of
the States themselves.
These questions admit of ready answers. If the interposition of the
general government should not be needed, the provision for such an
event will be a harmless superfluity only in the Constitution. But who
can say what experiments may be produced by the caprice of particular
States, by the ambition of enterprising leaders, or by the intrigues
and influence of foreign powers? To the second question it may be
answered, that if the general government should interpose by virtue of
this constitutional authority, it will be, of course, bound to pursue
the authority. But the authority extends no further than to a GUARANTY
of a republican form of government, which supposes a pre-existing
government of the form which is to be guaranteed. As long, therefore,
as the existing republican forms are continued by the States, they are
guaranteed by the federal Constitution. Whenever the States may choose
to substitute other republican forms, they have a right to do so, and
to claim the federal guaranty for the latter. The only restriction
imposed on them is, that they shall not exchange republican for
antirepublican Constitutions; a restriction which, it is presumed,
will hardly be considered as a grievance.
A protection against invasion is due from every society to the parts
composing it. The latitude of the expression here used seems to secure
each State, not only against foreign hostility, but against ambitious
or vindictive enterprises of its more powerful neighbors. The history,
both of ancient and modern confederacies, proves that the weaker
members of the union ought not to be insensible to the policy of this
article. Protection against domestic violence is added with equal
propriety. It has been remarked, that even among the Swiss cantons,
which, properly speaking, are not under one government, provision is
made for this object; and the history of that league informs us that
mutual aid is frequently claimed and afforded; and as well by the most
democratic, as the other cantons. A recent and well-known event among
ourselves has warned us to be prepared for emergencies of a like
nature. At first view, it might seem not to square with the republican
theory, to suppose, either that a majority have not the right, or that
a minority will have the force, to subvert a government; and
consequently, that the federal interposition can never be required,
but when it would be improper. But theoretic reasoning, in this as in
most other cases, must be qualified by the lessons of practice. Why
may not illicit combinations, for purposes of violence, be formed as
well by a majority of a State, especially a small State as by a
majority of a county, or a district of the same State; and if the
authority of the State ought, in the latter case, to protect the local
magistracy, ought not the federal authority, in the former, to support
the State authority? Besides, there are certain parts of the State
constitutions which are so interwoven with the federal Constitution,
that a violent blow cannot be given to the one without communicating
the wound to the other. Insurrections in a State will rarely induce a
federal interposition, unless the number concerned in them bear some
proportion to the friends of government. It will be much better that
the violence in such cases should be repressed by the superintending
power, than that the majority should be left to maintain their cause
by a bloody and obstinate contest. The existence of a right to
interpose, will generally prevent the necessity of exerting it.
Is it true that force and right are necessarily on the same side in
republican governments? May not the minor party possess such a
superiority of pecuniary resources, of military talents and
experience, or of secret succors from foreign powers, as will render
it superior also in an appeal to the sword? May not a more compact and
advantageous position turn the scale on the same side, against a
superior number so situated as to be less capable of a prompt and
collected exertion of its strength? Nothing can be more chimerical
than to imagine that in a trial of actual force, victory may be
calculated by the rules which prevail in a census of the inhabitants,
or which determine the event of an election! May it not happen, in
fine, that the minority of CITIZENS may become a majority of PERSONS,
by the accession of alien residents, of a casual concourse of
adventurers, or of those whom the constitution of the State has not
admitted to the rights of suffrage? I take no notice of an unhappy
species of population abounding in some of the States, who, during the
calm of regular government, are sunk below the level of men; but who,
in the tempestuous scenes of civil violence, may emerge into the human
character, and give a superiority of strength to any party with which
they may associate themselves. In cases where it may be doubtful on
which side justice lies, what better umpires could be desired by two
violent factions, flying to arms, and tearing a State to pieces, than
the representatives of confederate States, not heated by the local
flame? To the impartiality of judges, they would unite the affection
of friends. Happy would it be if such a remedy for its infirmities
could be enjoyed by all free governments; if a project equally
effectual could be established for the universal peace of mankind!
Should it be asked, what is to be the redress for an insurrection
pervading all the States, and comprising a superiority of the entire
force, though not a constitutional right? the answer must be, that
such a case, as it would be without the compass of human remedies, so
it is fortunately not within the compass of human probability; and
that it is a sufficient recommendation of the federal Constitution,
that it diminishes the risk of a calamity for which no possible
constitution can provide a cure. Among the advantages of a confederate
republic enumerated by Montesquieu, an important one is, "that should
a popular insurrection happen in one of the States, the others are
able to quell it. Should abuses creep into one part, they are reformed
by those that remain sound. ''
As a whole it makes very clear that this is an obligation of the federal government to the states and not a pact to ensure that the federal government remains republican (there are other parts of the constitution devoted to that issue). It also makes clear what a republican government is (one that is not aristocratic or a monarchy) as affirmed by Hamilton in Federalist Paper No. 84. And, it makes clear that the federal government can intervene to enforce this right (it exists so that there is a remedy for the right to a republican remedy that local military factions can't thwart).