The war of the American colonial settlers against British colonialism was progressive as it removed the fetters from the forces of production in North America. "Previous restrictions on trade and industry ended. As a result, an American merchant marine and manufacturing industry developed, especially in munitions and consumer products. … Large estates belonging to loyalist families were broken up into smaller plots. Primogeniture, the ancient British practice of passing on a family's entire estate to the eldest son, was ended. Both these policies provided increased opportunities for small, independent farmers.” 
At the same time, the war of the white colonists against the native nations was reactionary as its aim was to destroy and steal their land, not to help the them develop the forces of production.
"It is important to recognize that European and U.S. settler colonial projects unleashed massively destructive forces on Native peoples and communities. These include violence resulting directly from settler expansion, intertribal violence (frequently aggravated by colonial intrusions), enslavement, disease, alcohol, loss of land and resources, forced removals, and assaults on tribal religion, culture, and language. The configuration and impact of these forces varied considerably in different times and places according to the goals of particular colonial projects and the capacities of colonial societies and institutions to pursue them. The capacity of Native people and communities to directly resist, blunt, or evade colonial invasions proved equally important.” 
In the American civil war Marx and Engels supported the North against the South because capitalism, at that time, was still progressive and victory for the North would abolish slavery. They saw the war as continuation of the revolutionary war for independence. Of course this was not a war that could lead to socialism but they believed that victory for the North would advance the cause of all workers, both white and black, by destroying chattel slavery.
At the same time the sympathy of many in the British upper class, especially those who dealt with cotton was with the South, despite the fact that Britain had abolished slavery within its territories in 1833. However, the mill workers of Manchester supported the North. Though they endured long hardship the workers of 'Cottonopolis' refused to spin cotton picked by American slaves and supported Lincoln's embargo. Lincoln acknowledged this sacrifice when he wrote to the workers of Manchester in 1863 saying that theirs was an act of "sublime Christian heroism, which has not been surpassed in any age or in any country." 
British capitalism was more advanced than American capitalism in the 1850-1890 period "In this phase of development the growth process was led not by the textile industry but by the expansion of coal-mining, iron- and steel-making, railway construction, ship building, and other branches of mechanical engineering, such as the manufacture of steam-engines, textile machinery, and machine tools.” 
At the same period during the earliest phases of industrialization. “American industry … was … characterized by … local [miniature] monopolies protected from competition with each other by high transportation costs. By 1850, the average industrial plant in the US employed only seven workers. A year later, more than half of all British industrial enterprises had five or fewer employees.” At mid-century, the internal American market for manufactures was still smaller than that of the British. 
Marx and Engels did not support British colonialism even when it was more-advanced capitalism. They examine wars from the point of view of the international working class that understood that British imperialism was already a reactionary force against progress in the direction of democracy and socialism. England was already a bastion of reaction during the French revolution when it fought France in order to restore the old feudal regime and prevent democratic revolutions in other countries. It was motivated by the need of stability of profit flow.
Marx and Engels supported not only the Irish and the Polish struggles for independence but the struggle of the oppressed Indian people against British colonialism. "Marx was almost the very first to grasp the true nature of the revolt. On June 30, 1857 he explained the fact that the Sepoy’s revolt of 1857 were the first to rise by the pertinent observation that the Indian Army happened to be ‘the first general center of resistance which the Indian people were ever possessed of’.” On July 28, 1857, he quoted with approval Disraeli’s remark on the previous day: "the Indian disturbance is not a military mutiny, but a national revolt". On July 31 1857, Marx asserted that what John Bull considers to be a military mutiny ‘is in truth a national revolt.’ 
"Britain had been trading in India since about 1600, but it did not begin to seize large sections of land until 1757, after the Battle of Plassey. This battle pitted 3,000 soldiers of the British East India Company against the 5,000-strong army of the young Nawab of Bengal, Siraj ud Daulah, and his French East India Company allies. Fighting began on the morning of June 23, 1757. Heavy rain spoiled the Nawab's cannon powder (the British covered theirs), leading to his defeat. The Nawab lost at least 500 troops, to Britain's 22. Britain took the modern equivalent of about US $5 million from the Bengali treasury, which financed further expansion. The East India Company traded in cotton, silk, tea, and opium. Following the Battle of Plassey, it functioned as the military authority in growing sections of India, as well. By 1770, heavy Company taxation and other policies had left millions of Bengalis impoverished. While British soldiers and traders made their fortunes, the Indians starved. Between 1770 and 1773, about 10 million people died of famine in Bengal, one-third of the population.” 
Wars in the Epoch of Imperialism
In this epoch of the decline of capitalism, war between two imperialist camps like WWI and WWII (except for the Stalinist Soviet Union that was a degenerated workers state and the semi-colonial nations fighting against imperialists like China vs. Japan) will destroy the forces of production and most importantly the lives of millions and millions of workers.
The International Monetary Fund writes: "By the end of World War II, much of Europe and Asia, and parts of Africa, lay in ruins. Combat and bombing had flattened cities and towns, destroyed bridges and railroads, and scorched the countryside. The war had also taken a staggering toll in both military and civilian lives. Shortages of food, fuel, and all kinds of consumer products persisted and in many cases worsened after peace was declared. War-ravaged Europe and Japan could not produce enough goods for their own people, much less for export. (…) By 1947, the United States had accumulated 70% of the world’s gold reserves. The United Kingdom had gone from being the world’s greatest creditor to the world’s greatest debtor. Countries had sold off most of their gold and dollar reserves, as well as their foreign investments, to pay for the war. What few reserves remained were now quickly running out. Trade deficits meant there was little hope of replenishing them.” 
Severe inflation plagued the weakened economies. By 1948, wholesale prices were 200% higher in Austria, 1,820% higher in France, and a massive 10,100% higher in Japan than they had been before the war. In 1948, the French government devalued the franc by 80%, making a 5,000 franc note practically worthless. In some countries like Germany, the monetary system collapsed. People resorted to barter, often using cigarettes as money. 
Thus the imperialist wars, no matter under what ideology or regime, are reactionary because they destroy the forces of production on a large scale for the profits of small number of families. The only correct position in such wars is revolutionary defeats to all the imperialist camps. 
Another type of war is a war between an imperialist state and colonies or semi-colonies, for example the US war against Iraq or Afghanistan or British imperialism against Argentina (the Malvinas war in 1982). In such a war the RCIT stands with the oppressed people of the semi-colonies regardless of what leadership they have and in this struggle. Building the revolutionary working class party must include using the tactic of the Anti Imperialist United Front. 
Trotsky wrote in On the Sino-Japanese War: "In the Far East we have a classic example. China is a semi-colonial country which Japan is transforming, under our very eyes, into a colonial country. Japan’s struggle is imperialist and reactionary. China’s struggle is emancipatory and progressive. But Chiang Kai-shek? We need have no illusions about Chiang Kai-shek, his party, or the whole ruling class of China, just as Marx and Engels had no illusions about the ruling classes of Ireland and Poland. Chiang Kai-shek is the executioner of the Chinese workers and peasants. But today he is forced, despite himself, to struggle against Japan for the remainder of the independence of China. Tomorrow he may again betray. It is possible. It is probable. It is even inevitable. But today he is struggling. Only cowards, scoundrels, or complete imbeciles can refuse to participate in that struggle."
Another type of war is a war between two semi-colonies like the war between Iraq and Iran or Ethiopia and Eritrea. Another type is a war where imperialists use local armies of semi-colonies for example the African Union Mission to Somalia, AMISOM, a U.S.-controlled military operation financed by Washington that provided political, intelligence and diplomatic cover. Somalia is a source of oil and other strategic interests for imperialism. In such a war we oppose the servants of imperialism and call to defend the country under attack.
 See on this e.g. RCIT: On the 100th Anniversary of the Outbreak of World War I: The Struggle against Imperialism and War. The Marxist Understanding of Modern Imperialism and the Revolutionary Program in Light of the Increasing Rivalry between the Great Powers, Revolutionary Uprisings, and Counterrevolutionary Setbacks, 25.6.2014, https://www.thecommunists.net/theory/struggle-vs-imperialism-war/
 See on this e.g. Michael Pröbsting: The Great Robbery of the South. Continuity and Changes in the Super-Exploitation of the Semi-Colonial World by Monopoly Capital. Consequences for the Marxist Theory of Imperialism, Vienna 2013, chapter 12 and 13, https://www.thecommunists.net/theory/great-robbery-of-the-south/; Michael Pröbsting: Liberation Struggles and Imperialist Interference. The failure of sectarian “anti-imperialism” in the West: Some general considerations from the Marxist point of view and the example of the democratic revolution in Libya in 2011, https://www.thecommunists.net/theory/liberation-struggle-and-imperialism/