Calls for military action were met with mixed responses when the matter went before Parliament. Foreign Secretary Lord Palmerston, a politician known for his aggressive foreign policy and advocacy for free trade, led the pro war camp. Palmerston strongly believed that the destroyed opium should be considered property, not contraband, and as such reparations had to be made for its destruction. He justified military action by saying that no one could "say that he honestly believed the motive of the Chinese Government to have been the promotion of moral habits" and that the war was being fought to stem China's balance of payments deficit. After consulting with William Jardine, the foreign secretary drafted a letter to Prime Minister William Melbourne calling for a military response. Other merchants called for an opening of free trade with China, and it was commonly cited that the Chinese consumers were the driving factor of the opium trade. The periodic expulsion of British merchants from Canton and the refusal of the Qing government to treat Britain as a diplomatic equal were seen as a slight to national pride. Few Tory or liberal politicians supported the war. Sir James Graham, Lord Phillip Stanhope, and William Ewart Gladstone headed the anti-war faction in Britain, and denounced the ethics of the opium trade. After three days of debate, the vote was taken on Graham's motion on 9 April 1840, which was defeated by a majority of only 9 votes (262 votes for vs 271 votes against). The Tories in the House of Commons thus failed to deter the Government from proceeding with the war and stop the British warships already on their way to China. The House of Commons agreed on 27 July 1840 to a resolution of granting £173,442 for the expenses of the expedition to China, long after the war with China had broken out.

Under strong pressure and lobbying from various trade and manufacturer associations, the Whig cabinet under Prime Minister William Melbourne decided on 1 October 1839 to send an expedition to China. War preparations then began.

In early November 1839, Lord Palmerston instructed Auckland, Governor General of India, to prepare military forces for deployment in China. On 20 February 1840 Palmerston (who remained unaware of the First Battle of Chuenpi in November 1839) drafted two letters detailing the British response to the situation in China. One letter was addressed to the Charles Elliot, the other to the Daoguang Emperor and the Qing government. The letter to the Emperor informed China that Great Britain had sent a military expeditionary force to the Chinese coast. In the letter, Palmerston stated that,

These measures of hostility on the part of Great Britain against China are not only justified, but even rendered absolutely necessary, by the outrages which have been committed by the Chinese Authorities against British officers and Subjects, and these hostilities will not cease, until a satisfactory arrangement shall have been made by the Chinese Government.

In his letter to the Charles Elliot, Palmerston instructed the commanders to set up a blockade of the Pearl River and forward to a Chinese official the letter from Palmerston addressing the Chinese Emperor. They were to then capture the Chusan Islands, blockade the mouth of the Yangtze River, start negotiations with Qing officials, and finally sail the fleet into the Bohai Sea, where they would send another copy of the aforementioned letter to Beijing. Palmerston also issued a list of objectives that the British government wanted accomplished, with said objectives being

Lord Palmerston left it to Superintendent Charles Elliot's discretion as to how these objectives would be fulfilled, but noted that while negotiation would be a preferable outcome, he did not trust that diplomacy would succeed, writing;

To sum up in a few words the result of this Instruction, you will see, from what I have stated, that the British Government demands from that of China satisfaction for the past and security for the future; and does not choose to trust to negotiation for obtaining either of these things; but has sent out a Naval and Military Force with orders to begin at once to take the Measures necessary for attaining the object in view.

The British view was that to tolerate the Chinese action might mean the destruction of British trade in the Far East and that, on the other hand, it might be possible by active intervention to compel the Chinese to open other ports to European trade and to shake off the monopoly of the Guangzhou merchants. In 1840 British ships-of-war appeared off the south-eastern coast of China and bombarded it.