Battle of the Tagus The Battle of the Tagus was a naval engagement that took place on 11 July 1831 at the mouth of the Tagus river , in Portugal . A French fleet attacked and subdued Portuguese fortifications at the entrance of the Tagus , with the aim to strong - arm the government of Miguel I into recognising the newly established Kingdom of the French . The destruction of the forts defending access to the Tagus and the arrival of French warships at Lisbon forced the Portuguese to cave in and comply with French demands . Background . The accession of King Miguel I to the throne of Portugal and abolition of the Constitutional Charter had put the country under the rule of an absolutist monarch . Liberals challenged this usurpation , and the struggle of the Liberal Wars ensued . The government of Miguel I was hostile to France , and became even more so when the popular insurrection of the July Revolution deposited the absolutist Bourbon king Charles X , and established a constitutional monarchy in which Louis - Philippe had become " King of the French " . Miguel Refused to recognise the Monarchy of July , while the French nor the British government recognised his . In early 1831 , a French citizen , Edmond Potentin Bonhomme was sentenced to public flogging , a fine and exile for allegedly profaning a church , a claim that French officials dismissed as " they had good reason to believe that many of the statements it comprehends , have no other foundation than the bigotry and malevolence of the Portuguese priesthood " . Several other French citizen were similarly detained in what the French government judged to be arbitrary manners . The French consul in Lisbon , Cassas , was tasked to present a diplomatic demarche to the Portuguese government , but he was rebuffed as not having the diplomatic credentials of an ambassador . Viscout d ' Asseca requested a mediation of the British Foreign minister , Viscount Palmerston , who advised the Portuguese to comply with French demands . A squadron under Captain de Rabaudy , comprising the 60 - gun frigate " Melpomène " and the 18 - gun brig " Endymion " , under Nonay , was sent to the mouth of the Tagus river to show the flag and give weight to the demands of the French consul . The arrived on 16 March 1831 . Upon her departure from Brest , " Melpomère " ran aground on Basse Beuzec , and had to double back , leaving " Endymion " to continue her route . When " Melpomère " finally arrived on 16 May 1831 , " Endymion " was already returning to France , with the consul aboard . Rabaudy Stated the purpose of his mission to Portuguese authorities and delivered the French ultimatum , demanding : These demands had to be met 48 hours after reception . Portuguese authorities played for time . In consequence , Rabaudy ' s " Melpomère " started blockading Lisbon by preying upon shipping leaving and entering the Tagus , though their crew were immediately freed and sent ashore . " Melpomère " was soon reinforced by a squadron under Captain Charmasson , of the 52 - gun " Sirène " , escorted by the 18 - gun corvettes " Diligente " and " Églé " , and the 20 - gun brig " Hussard " . While some of the ships cruised off the mouth of the Tagus , the others were detached to Azores to hunt for a Portuguese Miguelist division that blockaded the Liberal controlled Terceira Island ; on 3 June , these ships captured the 24 - gun corvette " Urania " and the 6 - gun fluyt " Oreste " , but they failed to locate the rest of the Miguelist forces , and thus returned to the mouth of Tagus . In spite of the French blockade and their undermining of its war effort against Liberals , the Portuguese government refused to meet French demands , and the treatment of the French prisoners even deteriorated . The French therefore decided to escalate the matter , and Henri de Rigny , French Minister of the Navy , appointed rear - Admiral Baron Albin Roussin to a 6 - sail of the line squadron . Roussin was a veteran of the Mauritius campaign of 1809 – 1811 , and had been honoured for his conduct at the Battle of Grand Port . The mission of Rear - admiral Roussin was to sail to Tagus , with his flag on the 90 - gun " Suffren " , and give the Portuguese authorities an ultimatum that demanded , in addition to that already given by Rabaudy , that the chief of the Portuguese police be destitued ; that all sentences cast upon French citizens be annulled ; and that indemnities be paid to compensate for the costs induced by the expedition . The ultimatum expired 48 hours from reception , at which point the French forces would force the entrance of the Tagus , sail to Lisbon and bombard the city . Departed from Brest , the squadron arrived off the Tagus on 25 June , and stayed there waiting for the division inbound from Toulon , under Read - admiral Hugon . On 31 , the Portuguese government mobilised the warship on the Tage , though only two frigates and smaller warships could be marshaled at once ; the strongest asset on Tage , the 74 - gun " Dom João VI " , needed over a month of work worth of repairs , and the 52 - gun frigate " Amazona " could not be readied before twenty days . This left only the 54 - gun " Diana " and the 44 - gun " Pérola " , with the 26 - gun " Dom João I " and " Lealdade " , to oppose the much stronger French squadron . In addition , all the ships that the Portuguese could muster were much under - handed . The condition deteriorated on 1 July when " Suffren " and " Melpomène " chased the " Lord Wellington " , a Portuguese merchantman inbound from Bahia ; " Lord Wellington " fled under Fort Santo António to benefit from its protection . When the French ship approached , the fort opened fire , and a gunnery exchange broke out while the ships ' boats were launched and captured the merchantman . The French bombardment silenced the fort , killing five or six soldiers , and severely wounding thirty . On 6 , Hugon ' s squadron arrived from Toulon . Being forced to inaction by unfavourable winds , Roussin attempted a last diplomatic demarche and on 10 , he sent the 18 - gun brig " Dragon " , under Commander Théodore Deloffre , to present a last ultimatum ; " Dragon " also carried letters for the ships of the Tagus and for foreign diplomatic missions in Lisbon , and was under strict orders not to remain at anchor more than 24 hours . The new ultimatum comprised the same condition as the first , but added that the Portuguese government should compensate France for the price of the naval expedition , and threatened of a " de facto war " . " Dragon " returned to the French fleet the next day with a refusal , though Santarem attempted to surrender his French prisoners to the British as a token concession . Captain Vincent Moulac , of " Algésiras " , advised that the fleet attempt to force the entrance of the river and sail upstream to Lisbon . On 11 , the winds had turned , and a North - North - West wind allowed for the expedition upstream . The French squadron manoeuvred to form a line of battle , and at 13 : 30 , it sailed into the Southern pass . The French squadron sailed one by one in front of the forts , delivering broadsides at distances varying from 100 to 1000 metres , and reached Paço de Arcos in good order and with no serious damage . Roussin signaled his ships to sail on , but the two lead ships , " Marengo " and " Algésiras " , failed to spot the signals and dropped their anchor , as was initially planned to repair damage sustained during the forcing of the pass ; seeing that the flagship " Suffren " sailed before them without stopping , they promptly put to sail and took a place in the French line , but this mistake put " Suffren " at the front of the line . Around 16 : 00 , " Suffren " opened fire on the Belém Tower at a distance of 100 metres , soon followed by the rest of the squadron , and anchored in front of the Belém Palace while the other ships and frigates sailed on to the anchored Portuguese fleet . When the lead ship , " Pallas " , opened fire on the Portuguese squadron , all its ships struck , without firing a shot . By 17 : 00 , the entire French squadron was anchored before the docks of Lisbon , and Roussin sent his chief of staff , Lieutenant commander Charles Ollivier , to present the French ultimatum once again with a 2 - hour delay . The Portuguese government attempted once again to turn its prisoners over to British officials , but the offer was once again refused . The Portuguese government caved in , and a treaty was signed on 14 July on " Suffren " by Castello Branco and Roussin , with its dispositions published in the " Lisbon Gazette " on 15 , and the sums of money begin paid in cash by the 23 . With the ultimatum accepted by the Portuguese , Roussin sent most of the squadron to sea , staying in front of Lisbon with only " Suffren " , " Pallas " and " Melpomène " ; the 58 - gun frigate " Guerrière " , under Captain Kerdrain , rejoined him on 29 . Squadrons . Key Aftermath . The treaty of 14 July specified that the French would return the ships captured before the attack on the Tagus , including the warships " Orestes " and " Urania " and the merchantmen with their cargo . After a council of war on " Trident " , the French decided to also return the captured 74 - gun " Dom João VI " as , being neither armed nor manned during the action , she did not belong to an opposing force . Nothing was said , however , of the fleet captured at Lisbon itself . The Portuguese government protested , and Roussin proposed that the ships be re - purchased for 1 . 5 million cruzados , or to return half the captured ships , in exchange for the release of 400 political prisoners . When this offer was rejected , the ships in question were incorporated in the French fleet . The French squadron of Toulon departed on 25 ; the prizes " Diana " , " Amazone " , " Don João I " and " Don Pedro " departed with on 12 with a brig , and the next day " Perola " , " Lealdade " , " Saint Sebastian " and " Memoria " followed . The rest of the fleet eventually departed for Brest on 14 August , where it arrived on 4 September , leaving only " Melpomène " and " Églé " . In recognition for the deed , Roussin was promoted to Vice - admiral . The Miguelists would suffer a crushing defeat at the hands of the Liberals at the Battle of Cape St . Vincent two years later , ending the usurpation of Miguel I .