Theodore Krajensky was a Federation Ambassador in the 24th century. He was an old friend of Benjamin Sisko and considered by many as an expert on the Tzenkethi Coalition, but disliked the ruling Autarch.
In late 2371, Krajensky was scheduled for a vacation on Risa, but along the way, he was intercepted and murdered by a Changeling who took his place. The Changeling posing as Krajensky then went to Deep Space 9 and ordered newly promoted-Captain Sisko to take him into Tzenkethi space as a plan to cause a war between the two powers and weaken them as a prelude to the imminent Dominion War. However, the plot was discovered and the Changeling was accidentally killed by Odo. (DS9 episode: "The Adversary")
In an alternate timeline, Ambassador Krajensky was assigned to open peace talks with the Cardassians, working with Jean-Luc Picard and the crew of the USS Enterprise, in 2373. Krajensky would later bring evidence, gathered by Kira Nerys and Quark, to Federation President Jaresh-Inyo, that the Alpha Quadrant had been infiltrated by the Founders of the Dominion. After Jaresh-Inyo was revealed to be a Changeling and subsequently killed by Worf, Krajensky agreed with Charlie Whatley that a cover story should be put out. (ST novel: A Gutted World)