Literary response
Stein, Gary C. Indian Removal as Seen by European Travelers, Chronicles of Oklahoma 51 (Winter 1973-1974); 399-410.
Provides examples of European condemnation of the American policy and the American public's failure to criticize it.
Stein, Gary C. "And the Strife Never Ends": Indian-White Hostility as Seen by European Travelers in America, 1800-1860, Ethnohistory 20 (Spring 1973) 173-185
Numerous European travelers to America from 1800-1860observed and discussed the conflicts between Indians and white in the United States. Generally they wrote about the causes of these conflicts within the context pf life in America. Some displayed sympathy for the Indians, others saw him as a hopeless savage, and nearly all predicted his rapid disappearance.
Political and or elsewhere response
The physical European response was for immigrants from Europe to the U.S. to occupy the lands taken from the indigenous peoples by the U.S.
Large scale immigration resumed in the 1830s from Britain, Ireland,
Germany, and other parts of Central Europe as well as Scandinavia.
Most were attracted by the cheap farmland.
That "cheap farmland" is the land of the sovereign indigenous nations and peoples which the U.S. took by military force at the same time that the European immigrants were arriving in the U.S.
Between 1831 and 1840, immigration more than quadrupled to a total of
599,000.
Note the correlation between U.S. military actions against the indigenous nations of the U.S. which directly resulted in their property being taken by the U.S. and distributed to European immigrants.
Between 1841 and 1850, immigration nearly tripled again, totaling
1,713,000 immigrants, including at least 781,000 Irish, 435,000
Germans, 267,000 British, and 77,000 French.
The vertical expansion of specifically European immigration to the U.S. during the removal of native, indigenous (American Indian) from their sovereign national lands is no coincidence.
The indisputable physical European response that we have on the historical record was the act of vast numbers of Europeans becoming "immigrants" and "settlers" in the lands taken from the indigenous nations in the western hemisphere by U.S. military actions.
Source: History of immigration to the United States
The policy of removing indigenous peoples from their lands by force is not novel to European nation state or principality and subsequent U.S. history, whether that be in the form of direct military aggression, imminent domain, or so-called "race riots".
Note that there are several events in U.S. history which are referred to as the "Trail of Tears".
We can examine the prelude to the conquest of the indigenous nations and peoples in the western hemisphere via several documents composed by European powers before any of the "Trail of Tears" occurred to draw the conclusion that while individual European persons may have objected to the genocide of Native Americans, the policies of the European powers was decidedly conquest.
Romanus Pontifex
...since we had formerly by other letters of ours granted among other
things free and ample faculty to the aforesaid King Alfonso -- to
invade, search out, capture, vanquish, and subdue all Saracens and
pagans whatsoever, and other enemies of Christ wheresoever placed, and
the kingdoms, dukedoms, principalities, dominions, possessions, and
all movable and immovable goods whatsoever held and possessed by them
and to reduce their persons to perpetual slavery, and to apply and
appropriate to himself and his successors the kingdoms, dukedoms,
counties, principalities, dominions, possessions, and goods, and to
convert them to his and their use and profit -- by having secured the
said faculty, the said King Alfonso, or, by his authority, the
aforesaid infante, justly and lawfully has acquired and possessed, and
doth possess, these islands, lands, harbors, and seas, and they do of
right belong and pertain to the said King Alfonso and his successors,
nor without special license from King Alfonso and his successors
themselves has any other even of the faithful of Christ been entitled
hitherto, nor is he by any means now entitled lawfully to meddle
therewith.
Dum diversas
We grant you [Kings of Spain and Portugal] by these present documents,
with our Apostolic Authority, full and free permission to invade,
search out, capture, and subjugate the Saracens and pagans and any
other unbelievers and enemies of Christ wherever they may be, as well
as their kingdoms, duchies, counties, principalities, and other
property [...] and to reduce their persons into perpetual servitude.
Discovery doctrine
On the discovery of this immense continent, the great nations of
Europe ... as they were all in pursuit of nearly the same object, it
was necessary, in order to avoid conflicting settlements, and
consequent war with each other, to establish a principle which all
should acknowledge as the law by which the right of acquisition, which
they all asserted, should be regulated as between themselves. This
principle was that discovery gave title to the government by whose
subjects, or by whose authority, it was made, against all other
European governments, which title might be consummated by possession.
... The history of America, from its discovery to the present day,
proves, we think, the universal recognition of these principles.
Manifest Destiny
As the boundaries of America grew, white settlers and proponents of
expansion began to voice concerns over what they considered an
obstacle to settlement and America’s economic and social development –
the American Indian tribes living on lands east of the Mississippi
River which bordered white settlement. The land was home to many
tribal nations including the Cherokee, Creek and Seminole in the south
and the Choctaw and Chickasaw in the west. That land held the promise
of economic prosperity to raise cattle, wheat, and cotton, and harvest
timber and minerals. Eager to take possession of the land, the
settlers began to pressure the federal government to acquire the lands
from the Indian tribes. To these white settlers, the Indian tribes
were standing in the way of progress and of America’s manifest
destiny.
Monroe Doctrine
We owe it, therefore, to candor and to the amicable relations existing
between the United States and those powers to declare that we should
consider any attempt on their part to extend their system to any
portion of this hemisphere as dangerous to our peace and safety. With
the existing colonies or dependencies of any European power, we have
not interfered and shall not interfere.
The Debate over Indian Removal in the 1830s (Cass, 17, Kent, Commentaries, Vol. III, 310)
Cass argued that because Euro-American society was superior to North
American Indian society, civilized, Christian, settled agriculturists
had a God-given right to redistribute land occupied by barbarian,
heathen, wandering hunters. The established customs and practice of
colonizing countries from Europe supported Removal and past federal
laws and practice supported states such as Georgia in exercising
control over Indian communities and disposing of the land they
occupied. Cass said that clear Indian title to their land did not
really exist, and to support that quoted from arguments made before
the US Supreme Court. (Cass, Lewis. "Considerations of the Present State of the Indians and Their Removal to the West of the Mississippi," North American Review 30 (January 1830) reprint, New
York: Arno Press, 1975.) at 20
Cass also supported the assertion of Georgia’s right to claim Cherokee
land through an entitlement of Christians to take land of
non-Christians. He thought it was clear that being a heathen people
meant a loss of rights in the face of European claims.
Where the immigrant Europeans response was to become American "settler" of lands taken by the U.S. for those same arriving Europeans. The process was a symbiotic relationship. The U.S. is simultaneously stating that European powers where the immigrants were leaving should not interfere in the activities of the U.S. as to any U.S. policy in the western hemisphere.
The European response at that time was mixed, depending on whether the power was Britain or any other European power. Still, the actual people poured in to the U.S. It must be stressed here that the lands that were considered "cheap farmland" is the lands of the original peoples of that land which they were removed from by U.S. military force.
The historical facts are that a vast number of Europeans response was not to protest the removal of the sovereign, indigenous, native american peoples from their lands, but rather, to occupy those lands as "settlers", in effect, becoming instruments of the Indian removal policy themselves.
From the perspective of the native peoples, the entire enterprise of the several states was in effect the policy of Indian removal
John Ross (Kooneskoowe), Cherokee
... without the consent or knowledge of the native lords, a potentate of
England, whose eyes never saw, whose purse never purchased, and whose
sword never conquered the soil we inhabit, presumed to issue a
parchment, called a “Charter,” to the colony of Georgia, in which its
boundary was set forth, including a great extent of country inhabited
by the Cherokee and other Indian Nations.