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Middleton, 35, and Thevenet, 32 celebrated one year of being married on Sept. 12.
However, instead of posting a photo of him and his bride, Middleton paid a loving tribute to the late Queen Elizabeth. The picture he chose was one of the monarch on her wedding day with her husband, Prince Philip.
"Today is our 1st wedding anniversary," he wrote in the caption. "However it doesn't seem right to celebrate while mourning the loss of Queen Elizabeth II."
"Instead I'm sharing a photo of the marriage of the Queen & the Duke of Edinburgh on their wedding 74 years ago," his message continued.
He finished the note by connecting the longtime love between the royals, saying: "To my darling Alizee, I look forward to sharing as many with you in our lifetime ❤️."
As Kate Middleton's younger brother, James likely had opportunities to meet the Queen. The two shared a love of dogs, and Middleton shared a post featuring the Queen with her many pets, following the news of her death.
"Queen Elisabeth II, the epitome of steady grace, resolve and reassurance. Our constant in a world of change 🕊Thank you for a lifetime of service & leadership," his caption read.
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Middleton and Thevenet — a French financial analyst who is trilingual and grew up in Germany, Indonesia, Chile, France, Belgium and England — tied the knot last September, "surrounded by family, friends and of course a few dogs." Middleton shared the details in an Instagram post.
"Mr & Mrs Middleton ❤️ 🇬🇧 🇫🇷," he wrote. "Yesterday I married the love of my life surrounded by family, friends and of course a few dogs in the beautiful village [of] Bormes-les-Mimosas. Words cannot describe how happy I am ❤️."
Their dogs are a part of their love story. Middleton told The Telegraph that his cocker spaniel Ella approached Alizee at the South Kensington Club in the summer of 2018 when they were both having meetings.
"I'm truly so thankful to Ella for bringing me so much happiness into my life," the entrepreneur behind the dog food brand Ella & Co. said in an Instagram post. "She was the one who introduced me to Alizee, she supported me during my experience with clinical depression, she has kept me young & active and is the driving force behind my motivation to launch @ella.co."
They got engaged in 2019, when Middleton popped the question to Thevenet with a sapphire engagement ring. The ring appeared to be an emerald-cut sapphire in an Art Deco style and resembled Kate's iconic sapphire and diamond ring that was originally worn by Princess Diana.