Francisco Solano López Carrillo (24 July 1826 – 1 March 1870) was president of Paraguay from 1862 until his death in 1870. He was the eldest son of president Carlos Antonio López, whom he succeeded.

Considered ambitious, perhaps arrogant, and possibly insane, Solano López is widely regarded as being responsible for the War of the Triple Alliance, which led to his death.

Solano López was born in Manorá (Asunción). He was made Brigadier General of the Paraguayan army by his father at the age of 18, in 1844, during the spasmodic hostilities then prevailing with Argentina. He was sent in 1853 as minister plenipotentiary to Britain, France and Italy, and spent a year and a half in Europe. He purchased large quantities of arms and military supplies, together with several steamers, and organized a project for building a railroad and establishing a French colony in Paraguay. He also became infatuated with the empire of Napoleon III and Napoleon himself. López equipped his army with exact copies of uniforms of Napoleonic army. He ordered for himself an exact replica of Napoleon's crown. While there, he met Parisian courtesan Eliza Lynch and brought her with him back to Paraguay. There she was his mistress and de-facto first lady till his death, strongly influencing his later ambitious schemes.

Returning to Paraguay, he became Minister of War in 1855. Solano López was appointed as Vice President by his father. When his father died in 1862, López then called a congress that unanimously chose him as president for ten years.

In 1864, after Brazil began supporting a rising revolution in Uruguay by Venancio Flores against the White party government of Atanasio Aguirre, who was an ally of Paraguay and did not have friendly relations with Brazil, Uruguay requested Paraguay to intervene in the subject. López manifested this support via a letter to Brazil where it said that any occupation of Uruguayan lands by Brazil would be considered as an attack to Paraguay. When Brazil did not heed the letter and invaded Uruguay on October 12, 1864, López seized the Brazilian merchant steamer "Marqués de Olinda" in the harbour of Asunción and imprisoned the Brazilian governor of the province of Mato Grosso, who was on board. In the following month (December 1864) he dispatched a force to invade Mato Grosso, which seized and sacked the town of Corumbá and took possession of the province and its diamond mines. However, Paraguayan forces could not or would not seize the capital city of Cuiabá, in Northern Mato Grosso.

López next intended to send troops to Uruguay to support the government of his ally, yet when he requested from Argentina permission to cross Argentine soil, Argentine President Bartolomé Mitre refused to allow this force to cross the intervening province of Corrientes. López then declared war on Argentina. By then Brazil had already overthrown the Uruguayan government of Aguirre and installed Flores in his stead, rendering Uruguay little more than a Brazilian puppet. A hastily summoned congress composed of López's own nominees bestowed the title of Marshal upon him and gave him extraordinary war powers. On 13 April 1865, he declared war, seizing two Argentine war vessels in the Bay of Corrientes. The next day, he occupied the town of Corrientes, instituted a provisional government of his Argentine partisans, and announced that Paraguay had annexed Corrientes Province and Entre Ríos Province. On 1 May, Brazil joined Argentina and (conquered) Uruguay and signed the Treaty of the Triple Alliance, which stipulated that they should unitedly pursue the war until the existing government of Paraguay was overthrown, "until no arms or elements of war should be left to it." This agreement was literally carried out.

The war which ensued, lasting until 1 March 1870, was carried on with great stubbornness and with alternating fortunes, though López's disasters steadily increased. On September 12, 1866, López invited Mitre to a conference in Yatayty Corá. López had realized that the war was lost and was ready to sign a peace treaty with the Allies. No agreement was reached though since Mitre's conditions for rendition were that every article of the Secret Treaty of the Triple Alliance was still to be carried out, a condition to which López refused. Regardless of López refusal, a peace treaty was not something Mitre could guarantee since article 6 in the secret treaty stated that "The allies solemnly commit themselves no to abandon arms unless commonly agreed, and as long as they haven’t overthrown the current government of Paraguay, as well as not to try separately, nor sign any peace treaty, truce, armistice which would put an end to or suspend the war, unless agreed by all parties". This not only rendered any possibility of truce or peace nearly impossible but also stipulated that the war was to continue until the current government ceased to be, which meant the death of López.

In 1868, when the allies were pressing him hard, he convinced himself that his Paraguayan supporters had actually formed a conspiracy against his life. Thereupon several hundred prominent Paraguayan citizens were seized and executed by his order, including his brothers and brothers-in-law, cabinet ministers, judges, prefects, military officers, bishops and priests, and nine tenths of the civil officers, together with more than two hundred foreigners, among them several members of the diplomatic legations (the San Fernando massacres). During this time he also had his 70-year old mother flogged and ordered her execution, because she revealed to him that he was born out of wedlock. López also attempted to have himself canonized by the local bishops. In 1870 he proclaimed himself a saint. 23 Paraguayan bishops did not agree with the canonization and were executed on López's orders.

López was at last driven with a handful of troops to the northern frontier of Paraguay. He arrived at Cerro Corá on 14 February 1870. Two detachments were sent in pursuit of Solano López, who was accompanied by 200 men in the forests in the north where he received news of the considerable Brazilian force that were closing in on him. This caused some of the officials who were still with López to abandon him and approach the allied force, under the command of the Brazilian General José Antônio Correia da Câmara, which they readily joined as scouts in order to lead them to López. Upon hearing about this, López called a last war council with the remaining officers of his general staff in order to decide the course of action for the upcoming battle, whether they should escape into the hill range or stay and make a stand against the attackers. The counsel decided to stay and end the war once and for all by fighting to death.

The Brazilian force reached the camp on the 1 March. During the battle that ensued, López was separated from the remainder of his army and was accompanied only by his aide and a couple of officers. He had been wounded with a spear in the stomach and hit with a sword in the side of his head and so was too weak to walk by himself. They led him to the Aquidabangui stream. There the officers left López with the pretext of getting reinforcements. While he was alone with his aide, General Câmara arrived along with six soldiers and approached López offering him to surrender and guaranteeing his life. López refused and shouting, “Muero con mi patria!” (I die with my nation), tried to attack Câmara with his sword. Câmara ordered López to be disarmed, but López died during the struggle with the soldiers who were trying to disarm him. This incident marked the end of the war of the Triple Alliance.

There is a debate within Paraguay as to whether he was a fearless leader who led his troops to the end, or whether he foolishly led Paraguay into a war which it could never possibly win, and which nearly eliminated the country from the map. This debate was not helped by the revisionist stance taken by the Stroessner regime, regarding national history. Conversely, he is considered by some Latin Americans as a champion for the rights of smaller nations against the imperialism of more powerful neighbours. For example, Eduardo Galeano argues that he and his father continued the work of José Gaspar Rodríguez de Francia in defending Paraguay as "the only country that foreign capital had not deformed".

There is an ongoing debate in Paraguay among historians regarding López final words. The two versions are Muero por mi patria ("I die for my nation") or 'Muero con mi patria ("I die with my nation"). Regarding the event, Juansilvano Godoi wrote:

Marshal López died profoundly convinced that, along with him, the independence of Paraguay would disappear. He acquired this conviction upon learning that the allies had organized in Asuncion a "temporary government" made up of the Paraguayans who had taken arms against their government and fought for the Triple alliance army.

March 1 is a national holiday in Paraguay, called "Dia de los Heroes" (Heroes' Day), held in honour of López' memory. It is the most important holiday in the country next to the independence day. To date, López is considered to be the greatest Paraguayan national hero and his remains are located at the "Panteon de los Heroes" (National Pantheon of the Heroes) in Asuncion. It is customary in Asuncion that when something historic worth celebrating happens (such as the victory of the current President Lugo in the 2008 elections) people flock with their flags to the street in front of it and celebrate the event.

Recently, Argentine president Cristina Kirchner named an Argentinean military unit after Marshall Francisco Solano López. It was the 2nd Armored Artillery Group. During the ceremony the national anthem of both nations was sung and high ranking officers of both armies were present. The Chief of the Argentine Army gave a speech at the event saying the "Talking about the Paraguayan Army and the Argentine Army is talking of one and the same thing. Today, in the Argentine army, honored by the visit of Paraguay's Army Commandant, we are working intensely in fulfilling the dream of the fathers of our nation. Of those men who wanted to build a great nation, General San Martín and, precisely, Marshal López.". Afterwards, Lieutenant General Bendini said:

Marshall López inspired in his men a spirit and love for their land which made them prefer to die rather than surrendering. He is an example of what a leader is, a driver, a man who knows how to reach to his people. I am sure that the men of this artillery group will take the example of this brave Paraguayan soldier and will be deemed worthy of the name their unit carries.

At the end of the ceremony the Paraguayan Army Commandant presented the unit with a portrait of López.

The Paraguayan President's Office is called "Palacio de López" (Lopez's Palace) as it was built by López before the war as his home. However, it was never finished until after the war was over so he never resided in it.