On 25 May 1915, Asquith appointed Carson Attorney-General when the Coalition Government was formed after the Liberal government was brought down by the Shell Crisis and the resignation of Admiral Fisher.
He resigned on 19 October, however, citing his opposition to Government policy on war in the Balkans.
During Asquith's coalition government of 1915–1916, there was no formal opposition in either the Commons or the Lords.
The only party not in Asquith's Liberal, Conservative, Labour Coalition was the Irish Nationalist Party led by John Redmond.
However, this party supported the government and did not function as an Opposition.
After Carson, the leading figure among the Irish Unionist allies of the Conservative Party, resigned from the coalition ministry on 19 October 1915, he then became the de facto leader of those Unionists who were not members of the government, effectively Leader of the Opposition in the Commons.

He played a major role in forcing the resignation of Asquith as Prime Minister, returning to office on 10 December 1916 as First Lord of the Admiralty, and elevated to the powerful British War Cabinet as a Minister without Portfolio on 17 July 1917.

Carson was hostile to the foundation of the League of Nations as he believed that this institution would be ineffectual against war.
In a speech on 7 December 1917 he said:
Talk to me of treaties!
Talk to me of the League of Nations!
Every Great Power in Europe was pledged by treaty to preserve Belgium.
That was a League of Nations, but it failed.

Early in 1918, the government decided to extend conscription to Ireland, and that Ireland would have to be given home rule in order to make it acceptable.
Carson disagreed in principle and again resigned on 21 January.
He gave up his seat at the University of Dublin in the 1918 general election and was instead elected for Belfast Duncairn.

He continued to lead the Unionists, but when the Government of Ireland Act 1920 was introduced, advised his party to work for the exemption of six Ulster counties from Home Rule as the best compromise (a compromise he had previously rejected).
This proposal passed and as a result the Parliament of Northern Ireland was established.

In January 1921 he met in London over three days with Father O'Flanagan and Lord Justice Sir James O'Connor to try to find a mutual agreement that would end the Anglo-Irish war, but without result.

After the partition of Ireland, Carson repeatedly warned Ulster Unionist leaders not to alienate northern Catholics, as he foresaw this would make Northern Ireland unstable.
In 1921 he stated: "We used to say that we could not trust an Irish parliament in Dublin to do justice to the Protestant minority.
Let us take care that that reproach can no longer be made against your parliament, and from the outset let them see that the Catholic minority have nothing to fear from a Protestant majority."
In old age, while at London's Carlton Club, he confided to the Anglo-Irish (and Catholic) historian Sir Charles Petrie his disillusionment with Belfast politics: "I fought to keep Ulster part of the United Kingdom, but Stormont is turning her into a second-class Dominion."
On 25 May 1915, Asquith appointed Carson Attorney-General when the Coalition Government was formed after the Liberal government was brought down by the Shell Crisis and the resignation of Admiral Fisher. He resigned on 19 October, however, citing his opposition to Government policy on war in the Balkans. During Asquith's coalition government of 1915–1916, there was no formal opposition in either the Commons or the Lords. The only party not in Asquith's Liberal, Conservative, Labour Coalition was the Irish Nationalist Party led by John Redmond. However, this party supported the government and did not function as an Opposition. After Carson, the leading figure among the Irish Unionist allies of the Conservative Party, resigned from the coalition ministry on 19 October 1915, he then became the de facto leader of those Unionists who were not members of the government, effectively Leader of the Opposition in the Commons.
He played a major role in forcing the resignation of Asquith as Prime Minister, returning to office on 10 December 1916 as First Lord of the Admiralty, and elevated to the powerful British War Cabinet as a Minister without Portfolio on 17 July 1917.
Carson was hostile to the foundation of the League of Nations as he believed that this institution would be ineffectual against war. In a speech on 7 December 1917 he said:
Talk to me of treaties! Talk to me of the League of Nations! Every Great Power in Europe was pledged by treaty to preserve Belgium. That was a League of Nations, but it failed.
Early in 1918, the government decided to extend conscription to Ireland, and that Ireland would have to be given home rule in order to make it acceptable. Carson disagreed in principle and again resigned on 21 January. He gave up his seat at the University of Dublin in the 1918 general election and was instead elected for Belfast Duncairn.
He continued to lead the Unionists, but when the Government of Ireland Act 1920 was introduced, advised his party to work for the exemption of six Ulster counties from Home Rule as the best compromise (a compromise he had previously rejected). This proposal passed and as a result the Parliament of Northern Ireland was established.
In January 1921 he met in London over three days with Father O'Flanagan and Lord Justice Sir James O'Connor to try to find a mutual agreement that would end the Anglo-Irish war, but without result.
After the partition of Ireland, Carson repeatedly warned Ulster Unionist leaders not to alienate northern Catholics, as he foresaw this would make Northern Ireland unstable. In 1921 he stated: "We used to say that we could not trust an Irish parliament in Dublin to do justice to the Protestant minority. Let us take care that that reproach can no longer be made against your parliament, and from the outset let them see that the Catholic minority have nothing to fear from a Protestant majority." In old age, while at London's Carlton Club, he confided to the Anglo-Irish (and Catholic) historian Sir Charles Petrie his disillusionment with Belfast politics: "I fought to keep Ulster part of the United Kingdom, but Stormont is turning her into a second-class Dominion."
Edward Carson