f a population whose institutions were naturally antagonistic to
the slave idea. This being the case, she must be alarmed at seeing that
population steadily outstripping her own in numbers and wealth.[61]
Since she could not possibly even hold this disproportion stationary,
her best resource seemed to be to endeavor to keep it practically
harmless by maintaining a balance of power in the government. Thus it
became unwritten law that slave States and free States must be equal in
number, so that the South could not be outvoted in the Senate. This
system was practicable for a while, yet not a very long while; for the
North was filling up that great northwestern region, which was eternally
dedicated to freedom, and full-grown communities could not forever be
kept outside the pale of statehood. On the other hand, apart from any
question of numbers, the South could make no counter-expansion, because
she lay against a foreign country. After a time, however, Texas
opportunely rebelled against Mexico, and then the opportunity for
removing this obstruction was too obvious and too tempting to be lost. A
brief period of so-called independence on the part of Texas was followed
by the annexation of her territory to the United States,[62] with the
proviso that from her great area might in the future be cut off still
four other States. Slavery had been abolished in all Mexican territory,
and Texas had been properly a "free" country; but in becoming a part of
the United States she became also a slave State.

Mexico had declared that annexation of Texas would constitute a _casus
belli_, yet she was wisely laggard in beginning vindictive hostilities
against a power which could so easily whip her, and she probably never
would have done so had the United States rested content with an honest
boundary line. But this President Polk would not do, and by theft and
falsehood he at last fairly drove the Mexicans into a war, in which they
were so excessively beaten that the administration found itself able to
gather more plunder than it had expected. By the treaty of peace the
United States not only extended unjustly the southwestern boundary of
Texas, but also got New Mexico and California. To forward this result,
Polk had asked the House to place $2,000,000 at his disposal. Thereupon,
as an amendment to the bill granting this sum, Wilmot introduced his
famous proviso, prohibiting slavery in any part of the territory to be
acquired. Repeatedly and