By Fire and Sword (Polish: Ogniem i mieczem) is a historical novel by the Polish author Henryk Sienkiewicz, published in 1884. It is the first volume of a series known to Poles as The Trilogy, followed by The Deluge (Potop, 1886) and Fire in the Steppe (originally published under the Polish title Pan Wołodyjowski, which translates to Sir Wolodyjowski). The novel has been adapted as a film several times, most recently in 1999.

By Fire and Sword is a historical fiction novel, set in the 17th century in the Polish–Lithuanian Commonwealth during the Khmelnytsky Uprising. It was initially serialized in several Polish newspapers, chapters appearing in weekly installments. It gained enormous popularity in Poland, and by the turn of the 20th century had become one of the most popular Polish books ever. It became obligatory reading in Polish schools, and has been translated into English and most European languages.

The series was a vehicle for expressing Polish patriotism in a Poland partitioned and deprived of independence.

Despite some deviations, the book's historical framework is genuine and the fictional story is woven into real events. Many characters are historical figures, including Jeremi Wiśniowiecki and Bohdan Khmelnytsky. Sienkiewicz researched memoirs and chronicles of the Polish nobility, or szlachta, for details on life in 17th-century Poland. The book was written, according to the author, "to lift up the heart" of the Polish nation in the unhappy period following the failed January Uprising during the era of the partitions of Poland. Thus it often favors epic plots and heroic scenes over historical accuracy. Nonetheless, Sienkiewicz's vivid language made it one of the most popular books about that particular place and era.

Jan Skrzetuski, lieutenant of the armoured regiment of Prince Jeremi Wiśniowiecki, gives assistance to Bohdan Zenobi Chmielnicki (first posing as Abdank) as his party are returning from a mission to the Khan through the Wilderness. At Czyhryn the next day, Skrzetuski learns that Chmielnicki was escaping to the Sich. In a tavern he throws Czapliński, a voluble under–starosta (and Chmielnicki's deadly enemy), out through the door. It is here that he also becomes acquainted with Zagłoba and the Lithuanian Podbipięta, who wishes to join the service of Prince Jeremi in order to fulfil his family vow of cutting off the heads of three infidels, all at the same time with one blow.

On their way to Lubni, the party comes to the assistance of two women, one of whom is Helena Kurcewicz, returning to her aunt's home that really belongs to her. Jan's party are invited back to Rozlogi where Jan meets Bohun, a Cossack, adopted as a sixth son by the old princess (Helena's aunt). Bohun wants to pick a quarrel but is sent away and Jan is able to declare his love for Helena. Skrzetuski realizes the girl is being mistreated and denied her rights so gets the princess to promise Helena to him instead of Bohun or he will have Prince Jeremi help her recover home. The lieutenant finally arrives at Lubni and tells his comrades about his mission to the Crimea. Prince Jeremi returns and entertainments are laid on. To wile away the time, Skrzetuski fences with his friend, Michał Wołodyjowski, and receives a response to his letter sent to Helena via Rzędzian, his assistant.

Revolution is now afoot. The prince decides to send an envoy, in a group led by Pan (Sir) Bychowiec, to the Sich to find out about Chmielnicki. Jan persuades him to let him go in his place as he wants to see Helena and receives permission from the prince. He meets Helena once more at Rozlogi and then sets out for Czyhryn. Here, he meets Zaćwilichowski, an old colonel, who says he expects revolt in Ukraine. He also re-encounters Zagłoba who tells him that he has befriended Bohun. Further on his travels he decides to send Rzędzian with a message to Helena to flee the impending hordes. His party encounters some Cossacks and Tartars and a fight breaks out in which Jan's soldiers are slaughtered and he severely wounded. The alliance between the Cossacks and Tartars had been brokered by Chmielnicki, who understood that Cossacks, while having an excellent infantry, could not hope to match the Polish cavalry, the best in Europe. By combining Cossack infantry with Tartar cavalry, the uprising had balanced military force and a chance to beat the Polish army.

The messages to the friends in the court are discovered and two, Barabasz (Barabash) and Tatarczuk (Tatarchuk), are murdered by the Brotherhood of Cossacks. Tuhaj-bej (Tugay Bey), the Tartar leader, is given Jan as a ransom captive and news arrives that the Great Hetman, Mikołaj Potocki, has sent his son Stefan (Stephen) with his army against the Brotherhood, so Chmielnicki is chosen as their leader. The Zaporojians and Tartars march out of the Sitch to meet the Poles marching from Czyhryn. Chmielnicki avoids besieging Kudak. Krzeszowski, a Cossack, is sent to support Potocki but is won over by talks with Chmielnicki and massacres the German mercenaries who refuse to support his betrayal. At Żółte Wody (Zhovti Vody) the Polish hussars become bogged down in the soft mud and cannot attack on the second day of the battle, so Chmielnicki wins it and another at Kruta Bałka.