Special equipment: a small square of cheesecloth; a piece of butcher's twine
Make a sachet by putting the cinnamon sticks, star anise, juniper berries and cloves in the square of cheesecloth and tying it closed with butcher's twine.
Combine the wine, pomegranate juice, sherry and brown sugar in a large saucepan over medium heat. Bring to a simmer, reduce the heat to low, and cook gently, stirring occasionally, for 10 minutes. Remove from the heat and pour into a heatproof pitcher. Drop in the sachet and let it steep in the sangria steep for 10 minutes. Remove the sachet and discard.
To serve warm: Divide the orange and lemon slices among large mugs or heatproof glasses. Fill the glasses with warm sangria.
To serve cold: Put the sangria over an ice bath, stirring until cchilled, or refrigerate until chilled, about 2 hours. Pour the sangria into a large pitcher or punch bowl and stir in the orange and lemon slices. Serve in goblets over ice.