In 1855 Paraguay, with an understandable fear and distrust of its neighbours, began to develop the formidable Fortress of Humaitá. Established near the mouth of the River Paraguay and known as the Gibraltar of South America, it was the gateway to the country. However, it may have caused the Paraguayan government to feel itself invulnerable (wrongly, as it turned out), and it caused tension with Brazil. As explained by Lieutenant Colonel George Thompson of the Paraguayan Army: Between 1854 and 1864, and supervised by Francisco Solano López, Carlos Antonio López's son, Paraguay built up its military forces because it thought that the outside world (in particular Brazil, Argentina, the United States, and the British Empire) was disrespecting it. Paraguay imported some 200 foreign technicians, mostly British, and developed an iron foundry, an arsenal, a shipyard, a telegraph and a railway. All this modernisation was military. It also imported from Britain the cadre of a military medical corps who trained Paraguayan medical students. Thus Paraguay became an important regional power. However, as noted by John Hoyt Williams, "the dizzying speed with which his nation was 'modernized' and strengthened tempted López to flex his new muscle and seek an ever-expanding, activist role in international affairs". López became ruler of Paraguay in September 1862 after the death of his father. In 1862, the Uruguayan Blanco Party, traditionally hostile to Brazil, and now alarmed because its bitter enemies had seized power in Buenos Aires too, felt itself to be acutely short of allies. It began a covert diplomatic campaign to turn Paraguay aggressively against Buenos Aires. This campaign, which continued through 1864, insinuated that Buenos Aires had designs on both Paraguay and Uruguay, who should make a warlike alliance, detach Argentine provinces and seize Martín García Island. Although Francisco Solano López of Paraguay did not trust the Blancos and did not sign an alliance with Uruguay, he nevertheless built up his strength. Because women had always grown the subsistence crops in Paraguay, all able bodied men aged 16–50 were available for service and, in March 1864, they were conscripted for "intensive" military training. By September 1864 López had an estimated 48,000—150,000 men on a war footing, but anyway more than any other power in the region. Wrote Thomas L. Whigham: "Paraguay was the only country in the region that could boast of its military preparedness and a full treasury". However, as events transpired López