"After prolonged diplomatic fencing the famous Clayton-Bulwer Treaty was signed on April 19, 1850. By its terms both parties agreed to cooperate in the building of an Isthmian canal; both bound themselves never to fortify or exercise exclusive control over it. As regards clashing territorial ambitions in Central America, the pact was less clear. London was dead set against relinquishing the Mosquito Coast, and had no intention whatever of abandoning British Honduras. Secretary Clayton knew all this, yet he could not make specific concessions on these points for fear of arousing a partisan clamor from the Britain-haters in America. Both negotiators therefore deliberately consented to the use of ambiguous language to conceal their official differences; otherwise, a treaty probably could not have been concluded." Thomas A. Bailey, *A Diplomatic History of the American People (tenth edition), p. 275.
The result was the much-disputed Article I of the Clayton-Bulwer Treaty which provded that neither the US nor Great Britain would "occupy, or fortify, or colonize, or assume or exercise any dominion over Nicaragua, Costa Rica, the Mosquito coast, or any part of Central America..." http://avalon.law.yale.edu/19th_century/br1850.asp When Bulwer consented not to "occupy" any part of Central America, he understood that Britain was not to occupy anything *further.* Clayton, OTOH, hoped to make the provision retroactive. *The Times* (London) remarked that the negotiation was a struggle "for generalship in the use of terms." https://books.google.com/books?id=WFURAAAAYAAJ&pg=PA92
Anyway, the treaty eventually became one of the least popular treaties in US history because of the obstacle it presented to a US-run isthmian canal. In the 1880s, the proposed Frelinghuysen-Zavala Treaty for a US canal and virtual protectorate in Nicaragua was a clear-cut violation of the Clayton-Bulwer Treaty. (True, the Senate's vote for ratification was only 32-23, short of the needed two-thirds; but the majority was large enough to show that the 1850 commitment's days were numbered.) There was general relief in the US in 1901 when Clayton-Bulwer was finally nullified by the Hay-Pauncefote Treaty (to be more precise by the *second* Hay-Pauncefote Treaty; the first version of the treaty ran into trouble in the Senate because it denied the US the right to fortify the canal). In 1850, however, the Clayton-Bulwer treaty was ratified rather easily (42-11) by the Senate after only a brief debate. Even such a reputed Anglophobe as Lewis Cass voted for it.
According to Bailey, "this surprisingly speedy action is not difficult to account for. For one thing, the Senators had been consulted during the negotiations; for another, they were able to read their own meaning into the ambiguous language of the document. Before the final ratification, Clayton and Bulwer exchanged confidential notes which attempted to clarify their interpretations of what the treaty was really intended to accomplish. Clayton resorted to these under-the-table tactics primarily because he feared that the Senate would throw out the entire pact if the reservations were made public. At best, such stratagems were a questionable way of circumventing the regularly constituted treaty-making authority." Bailey, pp. 275-6.
(To quote an old Encyclopedia Americana article: "Before the ratifications were exchanged, however, Lord Palmerston wrote to Bulwer that the British government would interpret the treaty as not applying to [British] Honduras 'or its dependencies.' This could include Mosquitia, and it was Clayton's duty to settle that point before proceeding; but in fear of having his statesmanlike plan wrecked, and confident of no practical evil resulting, he assumed that it referred only to the islands, did not press Bulwer for assent to this construction, concealed the three quoted words from the Senate and the attorney-general, and the ratifications were exchanged."
http://books.google.com/books?id=f2tMAAAAMAAJ&pg=PT31)
Even in 1850, US support for the treaty was by no means unanimous. In particular, it was bitterly condemned by expansion-minded Democrats like Stephen Douglas and James Buchanan. If the Senators had been aware of Great Britain's interpretation of the treaty in its confidential notes to Clayton, the treaty would very likely not have been ratified (or ratified only with amendments or reservations that the British would have treated as a rejection). So let us say that the confidential notes are leaked, and the treaty is rejected. Consequences?
I don't think there would have been a British-US war in the 1850's, just as there was not in OTL over the divergent US and British interpretations of the treaty. To be sure, Americans were angry when the British not only refused to abandon the Honduran Bay Islands but made them a crown colony in 1852; and US-British relations became tense after the US bombardment and total destruction of Greytown in 1854 http://en.wikipedia.org/wiki/San_Juan_del_Norte which took no lives but destroyed the property of British subjects. The British, after their initial anger, took a moderate line; they dropped any demand for compensation from the US, and said that British subjects must seek their compensation from the Nicaraguan government which had tolerated the disorders which led to the bombardment. Of course this moderation may not have been unrelated to the fact that the Crimean War had broken out. William Walker's temporary seizure of Nicaragua complicated things further, but as Bailey notes (p. 278) "fortunately for relations between the two English-speaking nations, Walker's ambitious plans were finally ruined by disease, bad liquor, treachery, the opposition of Cornelius Vanderbilt's transit company, and British and French hostility."
IMO what prevented a British-American war over Central America in the 1850's was not the Clayton-Bulwer Treaty (which each party interpreted differently and accused the other of violating) but the fact that a war was not in either nation's interest. Great Britain had obvious naval supremacy, but was worried about the potential vulnerability of Canada, and of course there were also the troubles on the Continent. The US, especially after 1854, was largely preoccupied with the sectional dispute
over slavery, which made Northerners increasingly dubious about southward expansion. So despite all the tensions, having no violent changes in the status quo made sense. It is true, however, that the formal guarantee of the neutralization of any future isthmian canal did make it easier for the British to eventually abandon their Mosquito protectorate, since securing such a canal was the main objective of that protectorate. But even without a treaty it would be obvious that neither the US nor Britain would consent in the 1850s to sole control of a canal route by the other power, so the protectorate (which had come in for considerable criticism in Britain) would probably not have lasted long, anyway.
OTOH, after the ACW the treaty may have had a real effect, at least to the extent that I can see something like the Frelinghuysen-Zavala Treaty (perhaps a more modest version, without the protectorate provisions) being ratified in the 1880s if not for the Clayton-Bulwer Treaty. This was a transitional period, when the US felt much stronger relative to Great Britain in Central America than it had been in 1850 and therefore resented the restrictions of Clayton-Bulwer; OTOH, it was not yet strong enough (as it would be in 1901 with the Hay-Pauncefote Treaty) to more or less force Great Britain to abandon Clayton-Bulwer.
The result was the much-disputed Article I of the Clayton-Bulwer Treaty which provded that neither the US nor Great Britain would "occupy, or fortify, or colonize, or assume or exercise any dominion over Nicaragua, Costa Rica, the Mosquito coast, or any part of Central America..." http://avalon.law.yale.edu/19th_century/br1850.asp When Bulwer consented not to "occupy" any part of Central America, he understood that Britain was not to occupy anything *further.* Clayton, OTOH, hoped to make the provision retroactive. *The Times* (London) remarked that the negotiation was a struggle "for generalship in the use of terms." https://books.google.com/books?id=WFURAAAAYAAJ&pg=PA92
Anyway, the treaty eventually became one of the least popular treaties in US history because of the obstacle it presented to a US-run isthmian canal. In the 1880s, the proposed Frelinghuysen-Zavala Treaty for a US canal and virtual protectorate in Nicaragua was a clear-cut violation of the Clayton-Bulwer Treaty. (True, the Senate's vote for ratification was only 32-23, short of the needed two-thirds; but the majority was large enough to show that the 1850 commitment's days were numbered.) There was general relief in the US in 1901 when Clayton-Bulwer was finally nullified by the Hay-Pauncefote Treaty (to be more precise by the *second* Hay-Pauncefote Treaty; the first version of the treaty ran into trouble in the Senate because it denied the US the right to fortify the canal). In 1850, however, the Clayton-Bulwer treaty was ratified rather easily (42-11) by the Senate after only a brief debate. Even such a reputed Anglophobe as Lewis Cass voted for it.
According to Bailey, "this surprisingly speedy action is not difficult to account for. For one thing, the Senators had been consulted during the negotiations; for another, they were able to read their own meaning into the ambiguous language of the document. Before the final ratification, Clayton and Bulwer exchanged confidential notes which attempted to clarify their interpretations of what the treaty was really intended to accomplish. Clayton resorted to these under-the-table tactics primarily because he feared that the Senate would throw out the entire pact if the reservations were made public. At best, such stratagems were a questionable way of circumventing the regularly constituted treaty-making authority." Bailey, pp. 275-6.
(To quote an old Encyclopedia Americana article: "Before the ratifications were exchanged, however, Lord Palmerston wrote to Bulwer that the British government would interpret the treaty as not applying to [British] Honduras 'or its dependencies.' This could include Mosquitia, and it was Clayton's duty to settle that point before proceeding; but in fear of having his statesmanlike plan wrecked, and confident of no practical evil resulting, he assumed that it referred only to the islands, did not press Bulwer for assent to this construction, concealed the three quoted words from the Senate and the attorney-general, and the ratifications were exchanged."
http://books.google.com/books?id=f2tMAAAAMAAJ&pg=PT31)
Even in 1850, US support for the treaty was by no means unanimous. In particular, it was bitterly condemned by expansion-minded Democrats like Stephen Douglas and James Buchanan. If the Senators had been aware of Great Britain's interpretation of the treaty in its confidential notes to Clayton, the treaty would very likely not have been ratified (or ratified only with amendments or reservations that the British would have treated as a rejection). So let us say that the confidential notes are leaked, and the treaty is rejected. Consequences?
I don't think there would have been a British-US war in the 1850's, just as there was not in OTL over the divergent US and British interpretations of the treaty. To be sure, Americans were angry when the British not only refused to abandon the Honduran Bay Islands but made them a crown colony in 1852; and US-British relations became tense after the US bombardment and total destruction of Greytown in 1854 http://en.wikipedia.org/wiki/San_Juan_del_Norte which took no lives but destroyed the property of British subjects. The British, after their initial anger, took a moderate line; they dropped any demand for compensation from the US, and said that British subjects must seek their compensation from the Nicaraguan government which had tolerated the disorders which led to the bombardment. Of course this moderation may not have been unrelated to the fact that the Crimean War had broken out. William Walker's temporary seizure of Nicaragua complicated things further, but as Bailey notes (p. 278) "fortunately for relations between the two English-speaking nations, Walker's ambitious plans were finally ruined by disease, bad liquor, treachery, the opposition of Cornelius Vanderbilt's transit company, and British and French hostility."
IMO what prevented a British-American war over Central America in the 1850's was not the Clayton-Bulwer Treaty (which each party interpreted differently and accused the other of violating) but the fact that a war was not in either nation's interest. Great Britain had obvious naval supremacy, but was worried about the potential vulnerability of Canada, and of course there were also the troubles on the Continent. The US, especially after 1854, was largely preoccupied with the sectional dispute
over slavery, which made Northerners increasingly dubious about southward expansion. So despite all the tensions, having no violent changes in the status quo made sense. It is true, however, that the formal guarantee of the neutralization of any future isthmian canal did make it easier for the British to eventually abandon their Mosquito protectorate, since securing such a canal was the main objective of that protectorate. But even without a treaty it would be obvious that neither the US nor Britain would consent in the 1850s to sole control of a canal route by the other power, so the protectorate (which had come in for considerable criticism in Britain) would probably not have lasted long, anyway.
OTOH, after the ACW the treaty may have had a real effect, at least to the extent that I can see something like the Frelinghuysen-Zavala Treaty (perhaps a more modest version, without the protectorate provisions) being ratified in the 1880s if not for the Clayton-Bulwer Treaty. This was a transitional period, when the US felt much stronger relative to Great Britain in Central America than it had been in 1850 and therefore resented the restrictions of Clayton-Bulwer; OTOH, it was not yet strong enough (as it would be in 1901 with the Hay-Pauncefote Treaty) to more or less force Great Britain to abandon Clayton-Bulwer.