Leaving matters to be decided by Parliament was certainly safer, and also avoided the government taking the blame for anything. However, there was a huge downside: anything discussed in Parliament would be widely circulated and it was impossible to maintain relative secrecy.
In other words, the British Parliament might have just started voting on whether to declare war on the United States when the American ambassador in London would receive the news and then urgently contact the United States to prepare.
Such a scenario was simply unavoidable; after all, with so many eyes and ears in Parliament, it was impossible for any country to maintain complete secrecy.
Before submitting the proposal to Parliament, the British Government had urgently contacted many countries to confirm their opinions.
Among them, Britain valued France and Australasia the most, as these two countries were the strongest in the World Alliance apart from Britain itself.