Why did Henry Campbell-Bannerman become Prime Minister?

Question

Henry Campbell-Bannerman, leader of the Liberal party, became Prime Minister of the United Kingdom in 1905. However, the election which brought his party to power was in 1906. So why was he Prime Minister if the Conservatives were the largest party?

Answer 1

Technically you become prime minister by the king's appointment, rather than by parliamentary majority, and a century ago this meant more in reality than it does now. It could sometimes happen that if one party couldn't keep control of government  or parliament properly the prime minister would resign and the king might appoint the leader of the opposition as a sort of a caretaker before an election (for example the same thing happened with the Conservative Lord Salisbury when Gladstone resigned in 1885, Gladstone came back after the election a few months later). Arthur Balfour, the conservative PM, was unable to control his party due to massive divisions between protectionists and free-traders, as a consequence had to resign as PM a the end of 1905. Edward VII, a friend of Bannerman, put him and the liberals into government  for the month between Balfour's resignation and the January 1906 election, when they gained real power with that massive landslide victory.

Answer 2

After the disastrous "khaki election" of 1900, in which the Liberals lost control of Parliament, they were returned to a split  Parliament in 1905 but in a very divided state.  The Conservative Arthur Balfour formed a weak coalition government with some Trade Union and some Irish members.  

The coalition failed to carry votes in the House on 5th December 1905 and King Edward VII asked Campbell-Bannerman to form a minority administration with the caveat that he would go to the country within a year.  This he did very quickly in February 1906, and won one of the greatest Liberal victories of all times.

However, Campbell-Bannerman was 70 years old and had had a hard time holding the Liberals together since 1898.  He retired on grounds of ill health on 3rd April 1908 but remained resident at 10 Downing Street, pending the installation of a successor.  He died there on 22nd April 1908 and became the only (actual or former) Prime Minister to die at that address.