A golden cigarette case that King Rama VIII gave to former US president Franklin Roosevelt in 1945 to convey a covert message to Washington will serve among the highlights of an exhibition of royal gifts next month.
Running under the banner "Great and Good Friends", it will display 79 treasured presents that were handed to former US presidents by the Thai kings of the Chakri dynasty.
Most if not all of the gifts displayed from March 21 to June 30 at the Queen Sirikit Museum of Textiles have never been shown to the public before.
The exhibition is a marquee event commemorating 200 years of bilateral ties.
US Ambassador to Thailand Glyn Davies said the gifts show how friendships were formed starting from 1818.
Gifts from Their Majesties King Bhumibol Adulyadej and Queen Sirikit will be showcased, along with others from King Prajadhipok, King Rama VII; King Chulalongkorn, King Rama V; and King Mongkut, or Rama IV.
Presents from US presidents will fill out the collection.
Apart from the cigarette case, other highlights include a gold Niello desk set gifted to president Dwight D Eisenhower by King Bhumibol Adulyadej in 1960, and golden ceremonial robes that were presented to the Smithsonian by Prince Wan Waithayakorn.
Luang Praditmanutham, or Pridi Banomyong, jointly established the "Serithai" (Free Thai Movement) to work underground against the Japanese occupation during World War II.
Mr Davies said the cigarette case was his favorite piece from the exhibition.
Roosevelt was known to be an avid smoker and the gift contained a secret message about the Serithai movement.
Enclosed within was a message stating that the people of Siam -- Thailand's former name -- hoped to liberate the nation from the Japanese army.
Other pieces are also being displayed in Thailand for the first time, including items owned by the Smithsonian, the US National Archives and the Library of Congress, said Mr Davies.
Bilateral ties were forged in 1818 when an American ship returned to the US from Thailand carrying a letter addressed to US president James Monroe from then-foreign minister Dit Bunnag (Praja Surivongmontri).
The letter conveyed the wishes of King Rama II to establish trade between the two countries, marking the beginning of a lasting friendship.
Even though ties have chilled since the regime ascended to power in 2014 through a coup, Prime Minister Prayut Chan-o-cha visited the White House in October at the invitation of Donald Trump.