She then added a tribute to the letter - which was adorned with red poppies - in her own words.
“We will always remember the enormous sacrifice so many gave today and everyday. We write to honour the fallen men and women from across the world that gave their lives so that we could be free,” she wrote.
Adding the postscript, “Lest we forget, Eugenie.”
She also shared her post alongside several photos from the ceremony, including a photo that showed a group of veterans being pushed in wheelchairs beside a memorial covered in poppy wreaths.
Eugenie's personal tribute came as members of the royal family attended Remembrance Day services.
Meghan Markle, Prince Harry, Kate Middleton and Prince William all attended the ceremony, which was held at the Cenotaph war memorial in London.
Queen Elizabeth, Kate and Camilla, Duchess of Cornwall stood together at one balcony, while Meghan watched from the balcony of another window alongside Sophie, Countess of Wessex and Sir Timothy Laurence, Princess Anne‘s husband.
The Queen was overcome with emotion at the ceremony in London.
When Big Ben chimed at 11 am, the country observed two minutes of silence to remember those fallen in the line of duty.