The King of Siam offered Lincoln war elephants to aid in the war effort. For many reasons war elephants are not as useful in the age of steam and rifle, so president Lincoln said no. We didn't even accept them for our zoos.

To quote the national archives.

In a gesture of friendliness between the two countries, King Mongkut sent two gifts to President Buchanan during the last month of his administration—a sword and a photograph of the King with one of his children. In an accompanying letter, dated February 14, 1861, King Mongkut said that he had heard that the United States had no elephants. As a remedy, he offered a gift of elephants—several pairs of them—that could be "turned loose in forests and increase till there be large herds." The elephants would be useful in the unsettled parts of the United States, he continued, "since elephants being animals of great size and strength can bear burdens and travel through uncleared woods and matted jungles where no carriage and cart roads have yet been made." President Buchanan's successor, Abraham Lincoln, responded to the extraordinary offer. In a letter dated February 3, 1862, he graciously accepted the sword and photograph from the King but politely declined the elephants, explaining that the geography and climate of the United States do not "favor the multiplication of the elephant."

I have found the letter that we have sent in response to His Majesties great and generous offer.

To the King of Siam February 3, 1862

Abraham Lincoln, President of the United States of America.

To His Majesty Somdetch Phra Paramendr Maha Mongut,

King of Siam,

Great and Good Friend: I have received Your Majesty's two letters of the date of February 14th., 1861.

I have also received in good condition the royal gifts which accompanied those letters,---namely, a sword of costly materials and exquisite workmanship; a photographic likeness of Your Majesty and of Your Majesty's beloved daughter; and also two elephants' tusks of length and magnitude such as indicate that they could have belonged only to an animal which was a native of Siam. Your Majesty's letters show an understanding that our laws forbid the President from receiving these rich presents as personal treasures. They are therefore accepted in accordance with Your Majesty's desire as tokens of your good will and friendship for the American People. Congress being now in session at this capital, I have had great pleasure in making known to them this manifestation of Your Majesty's munificence and kind consideration.

Under their directions the gifts will be placed among the archives of the Government, where they will remain perpetually as tokens of mutual esteem and pacific dispositions more honorable to both nations than any trophies of conquest could be.

I appreciate most highly Your Majesty's tender of good offices in forwarding to this Government a stock from which a supply of elephants might be raised on our own soil. This Government would not hesitate to avail itself of so generous an offer if the object were one which could be made practically useful in the present condition of the United States.

Our political jurisdiction, however, does not reach a latitude so low as to favor the multiplication of the elephant, and steam on land, as well as on water, has been our best and most efficient agent of transportation in internal commerce.

I shall have occasion at no distant day to transmit to Your Majesty some token of indication of the high sense which this Government entertains of Your Majesty's friendship.

Meantime, wishing for Your Majesty a long and happy life, and for the generous and emulous People of Siam the highest possible prosperity, I commend both to the blessing of Almighty God.

Your Good Friend, ABRAHAM LINCOLN.

Washington, February 3, 1862.

By the President:

WILLIAM H. SEWARD, Secretary of State. Annotation

From my response to this question.

Secondly, the Russian fleet incident of 1863. There's a largely apocryphal tale that the Russians sent two fleets into US waters during the American Civil War, which would aid the Union in the event that the French or British intervened on the side of the Confederacy. The fleets were sent, it should be stressed, but the reason for it is less clear, and less likely to have been due to friendly and altruistic Russian support for the Americans. Thomas A. Bailey claims that the main reason that the ships were sent was so that the Tsar could confound his enemies, and certainly the Russians were able to benefit (for reasons mentioned below), with the sale of Alaska (1867) serving to further propagate the idea of Russian friendliness, which was then to lead into good Russo-American relations for much of the rest of the century.