What's the core idea of historical materialism which the Communist Manifesto starts out with? All history hitherto is the history of class struggle. We're only developing new scales for old methods of exploitation. Romans lacked heavy industry but they had factories with a division of labour reflecting them. They lacked the modern economic relationships that created the bourgeoisie but their patricians served the same role in society. Their colonialism was driven by primitive accumulation, a core population backed by a powerful military and industrial/logistical network which demands ever-greater levels of comfort. They needed Egyptian grain to distribute Caesar's bread, they needed British tin for bronze in regions without tin deposits.

Capitalism evolved out of the new division of labour that developed when aristocracy gave way to a merchant and bureaucrat class. A lord and serf had the same boss-employee relationship in a form reflecting the village and castle, but merchants could afford larger factories backed by larger resource extractors and the way power was wielded shifted away from the old institutions. The forms of modern colonialism increasingly became shaped by the bourgeoisie that profited from it instead of the older aristocratic forms. It's just a more complex manifestation of what has been happening since the Sumerians.

I would say the easiest way to begin is to divorce the historical/political idea of empires from Lenin's ideas of imperialism. There is a reason he called it "the highest stage of capitalism" after all. Capitalist imperialism is drastically different from pre-capitalist imperialism.

You can however, supported by historical reading, begin to see similar trends in the politics and economics of past empires, and ironically empires make a lot more sense as a political unit when you think about them from a materialist perspective. Why did the Romans and the Parthians fight over Armenia? Did crusaders invade the Levant to secure the holy land for pilgrimage or for Italian spice merchants? Why did the Mongols conquer so much? As happybadger says in his reply, this is just history repeating itself, but I would posit that it might be a good idea to firmly understand imperialism as a stage of capitalism (as it is now) first before you try to dig into the imperialism of historical empires because it will be confusing to try to understand imperialism as a universal phenomenon through different modes of production.

Smedley Butler's War is a Racket shows it in the context of US imperialism in Latin America. The military invaded countries for fruit companies. Countries were colonised for plantations as happened in the British Raj with tea. That marriage between corporation and state went from that to the modern military-industrial complex which drives the entire political system to boost aerospace companies and shipyards.

The Short answer: the bourgeois are parasitic sociopaths who do not like having their ambitions limited. If Capital rose at the regional level they would form societies to pressure their regional society at the political/ideological and cultural level to demand expansion to the national level. (buy politicians/buy up media and create cultural content displaying a militaristic outlook celebrating conquest or you know....Lightbearers of Democracy)

Once at the national level they would demand to be able to expand at the international level.

If they were absolutely restricted by expansion you would see Communist revolution break out. Russia for instance had a very weak bourgeois in 1917 and most of the industries in Russia were already captured by British and French capitalists. They could not expand. The Russian people therefore had the only option of to sink further into ruin or take a new weapon at the bourgeoisie

The long answer: Imperialism is not just a policy nor economics nor just politics. It is an entire encompassing system including economics, politics, ideology and culture.

The imperialists believe in a surplus reserve army of labour that is increasingly threatened with homelessness and pauperism. These parasites believe in a society of hierarchy where they are at the top and the vast mass of humanity scrabble in the dirt. Marx explains better:

The greater the social wealth, the functioning capital, the extent and energy of its growth, and, therefore, also the absolute mass of the proletariat and the productiveness of its labour, the greater is the industrial reserve army. The same causes which develop the expansive power of capital, develop also the labour power at its disposal. The relative mass of the industrial reserve army increases therefore with the potential energy of wealth. But the greater this reserve army in proportion to the active labour army, the greater is the mass of a consolidated surplus population, whose misery is in inverse ratio to its torment of labour. The more extensive, finally, the lazarus layers of the working class, and the industrial reserve army, the greater is official pauperism. This is the absolute general law of capitalist accumulation. Like all other laws it is modified in its working by many circumstances, the analysis of which does not concern us here. The folly is now patent of the economic wisdom that preaches to the labourers the accommodation of their number to the requirements of capital. The mechanism of capitalist production and accumulation constantly effects this adjustment. The first word of this adaptation is the creation of a relative surplus population, or industrial reserve army. Its last word is the misery of constantly extending strata of the active army of labour, and the dead weight of pauperism. The law by which a constantly increasing quantity of means of production, thanks to the advance in the productiveness of social labour, may be set in movement by a progressively diminishing expenditure of human power, this law, in a capitalist society – where the labourer does not employ the means of production, but the means of production employ the labourer – undergoes a complete inversion and is expressed thus: the higher the productiveness of labour, the greater is the pressure of the labourers on the means of employment, the more precarious, therefore, becomes their condition of existence, viz., the sale of their own labour power for the increasing of another’s wealth, or for the self-expansion of capital. The fact that the means of production, and the productiveness of labour, increase more rapidly than the productive population, expresses itself, therefore, capitalistically in the inverse form that the labouring population always increases more rapidly than the conditions under which capital can employ this increase for its own self-expansion.

We saw in Part IV., when analysing the production of relative surplus-value: within the capitalist system all methods for raising the social productiveness of labour are brought about at the cost of the individual labourer; all means for the development of production transform themselves into means of domination over, and exploitation of, the producers; they mutilate the labourer into a fragment of a man, degrade him to the level of an appendage of a machine, destroy every remnant of charm in his work and turn it into a hated toil; they estrange from him the intellectual potentialities of the labour process in the same proportion as science is incorporated in it as an independent power; they distort the conditions under which he works, subject him during the labour process to a despotism the more hateful for its meanness; they transform his life-time into working-time, and drag his wife and child beneath the wheels of the Juggernaut of capital. But all methods for the production of surplus-value are at the same time methods of accumulation; and every extension of accumulation becomes again a means for the development of those methods. It follows therefore that in proportion as capital accumulates, the lot of the labourer, be his payment high or low, must grow worse. The law, finally, that always equilibrates the relative surplus population, or industrial reserve army, to the extent and energy of accumulation, this law rivets the labourer to capital more firmly than the wedges of Vulcan did Prometheus to the rock. It establishes an accumulation of misery, corresponding with accumulation of capital. Accumulation of wealth at one pole is, therefore, at the same time accumulation of misery, agony of toil slavery, ignorance, brutality, mental degradation, at the opposite pole, i.e., on the side of the class that produces its own product in the form of capital.25

Marx, Capital, The General Law Of Capital Accumulation, P.798-799

Except for by the time Imperialism came on the scene the bourgeoisie were no longer content with labourers being "mutilated into an appendage of a machine" they were forcing them into trenches to fight other labourers and die by the millions - 125 million dead by counting just ww1 and 2 alone.

Stalin talked about how with the success of the October revolution it smashed part of the ideology in mens minds regarding race superiority which allowed genocides and atrocities that occurred in the colonies to continue

It was formerly the "accepted" idea that the world has been divided from time immemorial into inferior and superior races, into blacks and whites, of whom the former are unfit for civilization and are doomed to be objects of exploitation, while the latter are the only bearers of civilization, whose mission it is to exploit the former. That legend must now be regarded as shattered and discarded. One of the most important results of the October Revolution is that it dealt that legend a mortal blow, by demonstrating in practice that the liberated non-European peoples, drawn into the channel of Soviet development, are not one whit less capable of promoting a really progressive culture and a really progressive civilization than are the European peoples. It was formerly the "accepted" idea that the only method of liberating the oppressed peoples is the method of bourgeois nationalism, the method of nations drawing apart from one another, the method of disuniting nations, the method of intensifying national enmity among the labouring masses of the various nations.

That legend must now be regarded as refuted. One of the most important results of the October Revolution is that it dealt that legend a mortal blow, by demonstrating in practice the possibility and expediency of the proletarian, internationatist method of liberating the oppressed peoples, as the only correct method; by demonstrating in practice the possibility and expediency of a fraternal union of the workers and peasants of the most diverse nations based on the principles of voluntariness and internationalism. The existence of the Union of Soviet Socialist Republics, which is the prototype of the future integration of the working people of all countries into a single world economic system, cannot but serve as direct proof of this.

The era of tranquil exploitation and oppression of the colonies and dependent countries has passed away. The era of liberating revolutions in the colonies and dependent countries, the era of the awakening of the proletariat in those countries, the era of its hegemony in the revolution, has begun. 3. Having sown the seeds of revolution both in the centres of imperialism and in its rear, having weakened the might of imperialism in the "metropolises" and having shaken its domination in the colonies, the October Revolution has thereby put in jeopardy the very existence of world capitalism as a whole.

-J V Stalin, The International Character Of the October Revolution, 1927

Capital that does not expand gets bought up by capital that does. For example, wrestling was a regional thing in the 20th century. There was no massive brand and televised bouts between dudes in underwear prior to the WWE's rise to the top.

Wrestling promotions that did not expand like the WWE got bought up by them or pushed out by their overwhelming growth. If you wanted to see two top fighters in their promos fight like it was Batman vs Superman, you might never get the chance to. It was rare to cross promos, yet when the McMahon's stepped in you get Superman vs Batman every fuckin week.

I'm sure you see where this is going. In order to make more profits, the McMahon's ate all the wrestling promos they could. When that money ran out, they went worldwide to do shows wherever they can like Saudi Arabia. When that started to run out too they had to take their stars' side money and make them do more shit in their free time to feed the corporate beast.

Google Paige WWE Twitch and you'll get what I mean.

If the economy isn't expanding it means something has disrupted the circulation-reproduction-accumulation cycle of capital and you have an intense crisis on your hands.