In the presidential proclamation for Thanksgiving Day in 1863, President Abraham Lincoln had this to say:
"It is the duty of nations as well as of citizens to owe their dependence upon the overruling power of God; to confess their sins and transgressions in humble sorrow, yet with assured hope that genuine repentance will lead to mercy and pardon; and to recognize the sublime truth, announced in the Holy Scriptures and proven by all history, that those nations are blessed whose God is the Lord....
"We have been the recipients of the choicest bounties of heaven; we have been preserved these many years in peace and prosperity; we have grown in numbers, wealth and power as no other nation has ever grown. But we have forgotten God. We have forgotten the gracious hand which preserved us in peace and multiplied and enriched and strengthened us, and we have vainly imagined, in the deceitfulness of our hearts, that all these blessings were produced by some superior wisdom and virtue of our own. Intoxicated with unbroken success we have become too self-sufficient to feel the necessity of redeeming and preserving grace, too proud to pray to the God that made us.
"It has seemed to me fit and proper that God should be solemnly, reverently, and gratefully acknowledged, as with one heart and one voice, by the whole American people. I do therefore invite my fellow citizens in every part of the United States, and also those who are at sea and those who are sojourning in foreign lands, to set apart and observe the last Thursday of November as a day of Thanksgiving and praise to our beneficent Father who dwelleth in the heavens."
And a Thanksgiving prayer for the day, found elsewhere on the web:
Our Father in Heaven,
We give thanks for the pleasure
Of gathering together for this occasion.
We give thanks for this food
Prepared by loving hands.
We give thanks for life,
The freedom to enjoy it all
And all other blessings.
As we partake of this food,
We pray for health and strength
To carry on and try to live as You would have us.
This we ask in the name of Christ,
Our Heavenly Father.