Were they forced into it

They certainly were. Pius IX was the pontiff from 1846 until his death in 1878--basically the entirety of the process of Italian unification. At no point did he accept this process, and he resisted with armed force whenever possible. His armies were largely defeated in 1860-61, leaving only Rome under pontifical control. Rome finally fell in 1870, leaving the papacy in control of the Vatican (or what would later become the nation of Vatican City in 1929).

why didn't any catholic nations object?

Some did. For instance, Napoleon III stationed troops in Rome in the 1860's, partly to prop up the Papal States. Other nations had a number of people volunteer to serve the papacy in its military. However, the force of arms of the Italians was too much to bear. Napoleon III's troops stayed in Rome until 1870, and their presence was a deterrent because the nascent Italian state did not want to spark a war with its powerful French neighbor. This changed in 1870 with the beginning of the Franco-Prussian war, and due to a number of defeats Napoleon III withdrew his troops to attempt to help at home. Rome was left with only its small military, which gave only token resistance to the Italian attack. Rome was made the capital of Italy, and Pius IX declared himself a prisoner in the Vatican. At no point did he accept the situation, refusing to acknowledge Italian control (and since the papacy had been restored to its temporal power a number of times in the past this was not an entirely unreasonable position to take. Pius IX himself had been restored to Rome after the revolutions of 1848, for example). This restoration never happened, and "the Roman Question" lingered until the Lateran Accords were ratified in 1929.