US planners and senior civilian leadership were well aware of the importance as they saw it of containing the threat of the Axis, and of Germany and the European Axis as the greater immediate threat.

Following the re-militarization of Europe in the late 30's, while also the gathering storms in the Pacific, a new generation of joint, globally focused war plans were drawn up in 1938-39 by the Army and Navy. These so called 'Rainbow Plans' considered 5 broad options, starting with a focus on simply defending the homeland to start and no allies, to having France and Britain as allies, and either a focus on the Pacific or Europe first.

FDR had become firmly convinced of the incompatibility of Hitlerism with a peaceful, prosperous Europe, and as in 1914 that any general war in Europe would eventually drag the US in one way or another. And that a joint new effort by the Anglo-US-Franco nations offered the best chance at containing that threat, while also being the nations most likely to confront Imperial Japan in the Western Pacific.

Then in late 1940 in light of the threats of a global 2-Ocean Navy, and a navy(and army) far from ready to the task, and the need to establish coherent national strategy in the event of war a further important document is produced. Chief of Naval Operations Admiral Betty Stark writes a memo to FDR expanding upon the possibilities and looking at the Rainbow Plans, particularly #5 which assumed Germany First with European Allies.

Of all the options Stark laid out he supported and thought victory most likely for D(hence the name the Plan Dog Memo) which set forth a strong buildup of homefront defense, a focus on Germany first, and strong support to Britain through aid. FDR received this in November 1940 and broadly agreed with it on all points. That September had also of course seen the Destroyers for Bases deal with 50 odd old Flush Deck DD's transferred to Britain and Canada in return for basing rights, and soon the more expansive and formal Lend-Lease deals would take shape beyond Cash and Carry.

So it was in this background of most military, and senior civilian leadership accepting that war was coming and the President supporting a Europe first policy that the British delegation arrived in secret to start more formal talks and strategy coordination. Beyond simply increasing aid as possible, FDR also began preparing for other moves to aid Britain short of outright war. Among those were sending in the Marines in June 1941 to take over for the British force occupying Iceland, keeping it out of German hands, and freeing those forces to deploy elsewhere, and the Neutrality Patrol which sought to keep Axis combatant craft out of the Western Atlantic, and which begin escorting convoys to their halfway points to meet British escorts, and by that Fall would be in a shooting war with U-Boats. And set the groundwork for the Atlantic Conference that August where both Churchill and FDR would meet aboard warships in Newfoundland.

So while the general public remained staunchly divided if growing in unity against the axis short of outright war, senior leadership in the military and much of FDR's cabinet was not of that opinion, hence sneaking the British military delegation into DC. We can also say that secret prewar talks was nothing new. Notably before WW1 Churchill had engaged in talks with France over naval deployments, and he arguably well committed beyond his post Britain as First Lord of the Admiralty should have, but avoiding letting France down was then used as an argument for Britain getting involved in the war.

You may be interested in the US Army's official history of the prewar discussions too! It goes much more into the discussions of the service specific and joint planning boards and US analysis of British potential and management of the war. And some of those involved would go on to high posts! The Navy Captain Turner mentioned was Richmond Kelly Turner who was a favorite of Ernest King and would go on to command the Amphibious forces in the US(though FAR from free of controversy both personal and professional) https://history.army.mil/books/wwii/csppp/ch12.htm