Greetings! I shall attempt to give an overview of the various historiographical reasons why America was so helpful to the British and French before 1917, and why "straight up declar[ing] war on the Germans" was not as easy an option as it sounds. Do note however, that I am unable to respond on that whole "sending submarines through Canada" point, as my research has not yielded that specific nugget of information. Perhaps contacting the Great War series' editors and asking for their sources might be a good idea if it remains a topic of interests. Until then, let's begin.

When war was declared in 1914, America followed its foreign policy of "isolationism", in which it generally frowned upon and refused to take part in European affairs, especially if those affairs involved a conflict. This war seemed to be no different at the outset, and as a result the American public was overwhelmingly in support of neutrality. President Woodrow Wilson was not exactly passive in this regard either (that is, he did not simply declare US neutrality and leave it at that). He monitored the war as it unfolded "across the pond", and in 1916 his personal emissary Colonel Edward House attempted to explore the possibility of an Allied-German peace settlement, with America mediating the two sides. This of course, was not possible in a year which had seen so many (at least superficial) German victories on both the Western and Eastern Fronts, and would not satisfy the Anglo-French terms of peace either.

The American public itself was also divided by 1916. Whilst Wilson believed that "British navalism" was just as bad as "German militarism", there were two clear sides forming (albeit minority sides, as neutrality remained the major sentiment up until 1917). On the one hand, America was full of ethnic German-Americans, whose vote Wilson may have wished to secure by promising neutrality in the coming years. There were also Irish voters in the Eastern cities, whose fury at the British oppression during the 1916 Easter Uprising meant they were against the US joining an old enemy. On the other hand, the Pro-British side was formed of the 'Wasp" (White Anglo-Saxon Protestant, as Michael Howard terms it) supremacy on the east coast, as well as notable figures such as newspaper magnate William Randolph Hearst and ex-President Theodore Roosevelt. Neither side could claim the majority up until 1917, so America had to remain politically neutral in the whole affair.

Economically however, America was most certainly not neutral, though this was mostly due to wartime fact rather than commercial preference. France and Britain relied to a considerable extent on shipments of American goods to sustain the war effort, and the US was more than willing to extend credit to these countries and continue the flow of materiel. On the other hand, the Royal Navy's blockade of Germany meant that no US goods were able to flow into that country. This of course created a natural "tilt" towards the Entente Powers, and gave the traders in America more to worry about as the war dragged on. Michael Howard sums up this economical perspective well:

"Yet as the war went on an increasing amount of that business [America getting involved in war] consisted in supplying war material to the Allies - not necessarily out of ideological sympathy, but because they could not get it to the Germans. If that trade were interrupted, then the war would become their business, whether they like it or not."

This fear of interrupted trade had been realised in 1915 when the Lusitania sank with 128 American lives onboard. Though she had been carrying ammunition in a secret hold compartment, the Germans were forced to scale down their submarine offensives in the Atlantic and operate by "cruiser warfare rules", by which all passengers had to be warned of an attack, allowed to abandon ship, and pointed in the direction of the nearest port (idealistic at best, downright impossible and impractical at worst).

In 1917 however, all this changed. With the removal of Russia from the war, the American public began to realise that of the two sides in Europe, there was a lesser evil. They had seen propaganda posters and news reports of the horrific German atrocities in Belgium, the use of poison gas, and the POW camps. The LIFE magazine (a satirical one, not the one we have today) even circulated a cover in 1917 of a picture of the American states "if Germany won the war". Fascinating image by the way, worth a look and analysis here. So clearly in 1916-17 we have a clear lean of sympathies towards the Entente and the Anglo-French nations, but how did this translate to American involvement?

It all came to a climax in the Spring of 1917. We of course have the Zimmerman telegram which was published in the American press and read like a declaration of war (despite the fact it was anything but). We also have the German decision, after much internal and external discussion, to resume unrestricted submarine warfare. Once these two events had occurred, the fuse was lit. It was only a matter of Wilson deciding to declare war, which he very reluctantly did on April 5, 1917.

So, to sum up the critical points to this question:

Why was there such an effort to help the British? And why was it done in such secrecy when they could have just straight up declared war on the Germans?

The effort to help the British and French started off as a purely economical one, and that was in of itself a practical rather than ideological preference for trade (remember the RN blockade of Germany). America could not declare war without risking internal divisions and a serious political break from the previous decades of "anti-European involvement". It was in 1917 when a major growth of public support for the British, condemnation of the Germans (especially after the Zimmerman Telegram) , and the resumption of submarine warfare that pressured Wilson to go to war.

Let me know if you have any follow-ups and I hope you found this response useful!

Sources

Fordham, Benjamin O. "Revisionism Reconsidered: Exports and American Intervention in World War I." International Organization 61, no. 2 (2007): 277-310. Accessed January 24, 2021. http://www.jstor.org/stable/4498146.

Howard, Michael. The First World War: A Very Short Introduction. Oxford University Press, 2007.

Gompert, David C., Hans Binnendijk, and Bonny Lin. "Woodrow Wilson’s Decision to Enter World War I, 1917." In Blinders, Blunders, and Wars: What America and China Can Learn, 71-80. RAND Corporation, 2014. Accessed January 24, 2021. http://www.jstor.org/stable/10.7249/j.ctt1287m9t.13.

KENNEDY, ROSS A. "Woodrow Wilson, World War I, and an American Conception of National Security." Diplomatic History 25, no. 1 (2001): 1-31. Accessed January 24, 2021. http://www.jstor.org/stable/24913819.

Neiberg, Michael. "America and the World War, 1914-1917." Lecture delivered on 3 November, 2017. Accessible online here. (highly recommend watching this freely accessible lecture for a more in-depth exploration of the concepts in the response).

/u/Starwarsnerd222's answer is excellent, but I would like to tag on in regards to the submarines you had mentioned.

The United States government did not build submarines for the United Kingdom at that point. However, there was in fact a deal involving the Electric Boat Company (based in Groton, Connecticut), the Fore River Shipyard (in Quincy, Massachusetts), the Bethlehem Steel Corporation (Bethlehem, Pennsylvania) and the United Kingdom to build 20 H Class submarines based off of an Electric Boat design. 10 would eventually be built in the United States, and the other 10 would be built in Canada.

During this time period the United States Government did not operate its own shipyards for submarines or design its own submarines (the Navy would start designing its own submarines in 1915 and 1916). Rather, it put out contracts which specified design features that they wished to see in a class of submarines, and the price they expected to pay for the class. The two submarine design firms in the United States at the time were Electric Boat Company which was based in Groton, Connecticut and formed originally by Isaac Rice to build submarines designed by John Holland and the other was the Lake Torpedo Boat Company which was based in Bridgeport, Connecticut and founded by Simon Lake. These two firms would respond to these contracts with counter-offers, often with a more expensive price-tag and without all of the features that the U.S. Government wanted.^1 But, as the U.S. Government at this time did not the capacity to build its own submarines, they could only acquiesce to the firms.

So in the United States you have two independent submarine designers who are not part of the government and were not exclusively designing and selling submarines to the United States. They sold their designs and submarines around the globe. For example, In 1914, after the success of German submarine U-9 in sinking the HMS Hawke (U-9 had earlier sunk the cruisers Aboukir, Cressy, and Hogue), Simon Lake had this to say about the German designs:^2

The result was that I made an agreement with Krupps who were to build my type in Russia and Germany. I placed in the German hands my plans, copies of patents and technical data.

The German submarines in principle are practically duplicates of the Lake type in regard to submarine torpedo characteristics.

Now, the interview as a whole does have to be read, at least a little bit, as Lake attempting to sell the world on the military effectiveness of his boats. But, Lake did in fact have a deal with the Krupp firm to build his submarines which was in the words of one biographer “very lucrative”. Lake would receive a consulting fee from Krupp, and the Lake Torpedo Boat Company would receive 12 ½% royalties on Krupp built submarines for Russia, 7 ½% royalties on Krupp built submarines for Italy, and 6% royalties for Krupp built submarines for Germany.^3

This brings us to the United Kingdom. Similarly, Electric Boat had sold rights and designs for Holland designed boats to Vickers and Maxim to build them in the United Kingdom and the British Empire. In November 1914 an order for submarines was placed at the Fore River Shipyard in Quincy, Massachusetts through the Bethlehem Steel Corporation out of Pennsylvania. This initial order in November, 1914 was for 20 submarines, although they would not be completed in the United States, rather they would be shipped in parts to be assembled elsewhere, mainly Canada.

These would use the British H class designs, which were originally a design from Electric Boat. By December, Charles M. Scwhab, who ran the Bethlehem Steel Corporation, was being questioned by the Secretary of States William Jennings Bryan who pressured Schwab into cancelling the submarine orders.^4 It’s not entirely fair to say that Schwab “cancelled” the orders, rather ten of the submarines had all construction shifted to Canada to a Vickers & Maxim shipyard (they had not begun construction in the U.S. yet), while the other ten were launched but interned by the U.S. Government until the U.S. joined the war. A number of them would be sold to Chile by the UK as a payment for their seizure of unfinished ships originally intended for Chile.

So in the case of these submarines, it wasn’t the United States government who sold them to the United Kingdom. The Government was actually against such a deal. Rather, it was private companies and shipyards who had worked out contracts worth upwards of $10,000,000.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

• 1: Gary E. Weir, Building American Submarines, page 12
• 2: “A Submarine Surprise: Simon Lake Thinks the Germans May Provide It”, The Hartford Courant, October 21st, 1914, page 19
• 3: John J. Poluhowich, Argonaut: The Submarine Legacy of Simon Lake, page 99.
• 4: “Schwab Back with Substitute Orders: Went Abroad to Cancel Submarine Contracts – Predicts Great Prosperity”, The Hartford Courant, December 24th, 1914 page 11.