The fall of the Liberal government in 1874, and its replacement with Benjamin Disraeli's Conservative administration meant that the repeal campaign stalled; Butler called it a "year of discouragement" when there was "deep depression in the work". Although the LNA kept up the pressure, progress in persuading Liberal MPs to oppose the Contagious Diseases Acts was slow, and the government was implacable in its support of the measures. At a meeting of regional LNA branches in May, one speech focused on legislation in Europe; the meeting resolved to correspond with sister organisations on the continent. At the start of December 1874 Butler left for Paris and a tour that covered France, Italy and Switzerland, where she met with local pressure groups and civic authorities. She encountered strong support from feminist groups, but hostility from the authorities. She returned from her travels at the end of February 1875. As a result of her experiences, in March 1875 Butler formed the British and Continental Federation for the Abolition of Prostitution (later renamed the International Abolitionist Federation), an organisation that campaigned against state regulation of prostitution and for "the abolition of female slavery and the elevation of public morality among men". The Liberal MP James Stansfeld--who wished to repeal the Acts--became the federation's first general secretary; Butler and her friend, the Liberal MP Henry Wilson, became joint secretaries. In 1878 Josephine wrote a biography of Catherine of Siena, which Glen Petrie--her biographer--thought was probably her best work; Walkowitz considers the work provided a "historical justification for her own political activism". Another biographer, Helen Mathers, believes that "in emphasising that she and Catherine were born to be leaders, of both men and women, ... [Butler] made a profound contribution to feminism". Butler became aware of the slave trade of young women and children from England to mainland Europe in 1879. Young girls were considered "fair game", according to Mathers, as the law allowed them to become prostitutes at the age of 13. After playing a minor role in starting an investigation into an accusation of trafficking, Butler became active in the campaign in May 1880, and wrote to The Shield that "the official houses of prostitution in Brussels are crowded with English minor girls", and that in one house "there are immured little children, English girls of from twelve to fifteen years of age ... stolen, kidnapped, betrayed, got from English country villages by every artifice and sold to these human shambles". She visited Brussels where she met the mayor and local councillors and made allegations against the head of the Belgian Police des Moeurs and his deputy as to their involvement in the trade. After the meeting she was contacted by a detective who confirmed that the senior members of the Police des Moeurs were guilty of collusion with brothel keepers. She returned home and filed a deposition containing a copy of the statement from the detective and sent them to the Procureur du Roi (Chief Prosecutor) and the British Home Secretary. Following an investigation in Belgium, the head of the Police des Moeurs was removed from office, and his deputy was put on trial alongside 12 brothel owners; all were imprisoned for their roles in the trade. CANNOTANSWER

Answer this question "Why were they traded?" by extracting the answer from the text above.

the law allowed them to become prostitutes at the age of 13.