The Spanish-American War came about because the long decline of Spanish influence in the New World was culminating. In other words, Spanish colonies in South America and Caribbean had been breaking away from Spain from the 1820's onward, primarily because of the US's example of breaking away from England. Having lost its presence in Mexico and southern North America, first the country of Texas, followed by the US, expanded into those areas during the 1840's. By the time of the Spanish-American War in 1898, Spain had been nearly driven from the New World, and the US became the dominant power in the region. Are wars of expansion just?
It seemed as if the Spanish- American War was fought on the principles of freedom and statehood, and at the same time, seemed to deny them. If we apply the ideas of statehood and autonomy from the Colonial times to the situation at stake in this conflict, one sees some level of comparison present. There were issues of state sovereignty present and the idea of self- determination was present. American influence at the time was one where its own survival was not being directly threatened or challenged. The offensive on Cuba helped to strengthen American interests in the region. It was interesting to see how the same principles that guided America's drive for freedom could not be applicable to other nations striving for the same thing. In this sense, one could see the war as not necessarily being just.
This is hard to answer because it is not clear what a "just" war is.
It was clearly a morally good thing for Cuba to become independent of Spain. We assume that all countries deserve to rule themselves. So in that sense, it was just.
But the US did not actually allow Cuba to rule itself after the war (see Platt Amendment). In addition, the US took Puerto Rico and the Philippines (not to mention Guam). The Philippines, in particular, wanted independence and it took a very nasty war to suppress them. So in that sense it was not just.
Spanish American war took place between April and August 1898 on the issue of liberation of Cuba from oppressive rule of Spain. Though not the declared objective of the war, USA also won Puerto Rico, Guam and Philippine islands as a result of this war. Though the apparent objective of USA in fighting this war was to oppose Imperialistic rule in Cuba, a direct outcome of this war was USA trying to establish its rule over Philippine, which involved forcibly putting down opposition of people of Philippines. Also, the kind of relationship that developed between USA and Cuba, ans as it stands today, does not reflect very high concern on part of USA for freedom of Cuba.
In view of these facts, it is difficult to justify American actions in Spenish American war on moral grounds. However on the whole it did help USA by giving it the status of a world power. The war also resulted in USA initiating action to improve its defence and build facilities like Panama Canal, that have been very useful to USA in the long run.
What do you determine as 'just'? A just war needs to be proved the reason behind the war in the first place. I don't believe it was just because Spain did not make the first move in fact. There are many sides to this