You're right, but I chose the example of US Marines because much of their esteem is based on their ideal of rigourous self-selection. Even with the tawdriness of American social fabric, the psychology of a given Marine may have a greater chance of compatibility with another Marine than someone in the general populace. I mean that the ideal of the Marines - that each and every Marine is long-lost kin - is patently insane.
It is ridiculous because what makes a unified moral community is not empty abstractions or mere personality traits(two aggressive people can find themselves with very different moral beliefs), but rather shared beliefs about what is right and what is important for society to pursue. I still think Marines are more harmonious in this way that randomly selected Americans, but it remains a fairly shallow kinship.
The question of "how large" is indeed the pressing one. A smaller, more vigourously selected group will doubtless yield greater potential for moral compatibility. Because I argue that the most moral morality comes from direct relationships, I consider smallest best (each individual personally knows each other, or knows of each other); besides this, smaller is generally better. But there are many reasons why "smaller" is a chimera.
I generally agree, but I think we're confusing morality with morality as practiced. If everyone knows one another, they're more likely to practice their moral beliefs. On the other hand, suppose that there is a community in which everyone has basically the same values, but they don't all know one another. They will still have clear ideas about whether or not an action is right or wrong, but they will be more likely to make exceptions in their own cases because it's not a friend or relative on the other side of the interaction. There are a few points that we can infer from these points. First, morality itself depends upon shared moral norms. A society could be as huge and dispersed as the US and yet have a unified, coherent moral system, provided that citizens shared moral attitudes in common. Second, in the hypothetical scenario I have described, people would not behave as morally as they would in a smaller community which was identical morally. People have a tendency to go against their morals when those they're abusing are unknown to them. Third, we all know that the scenario is impossible. People distributed across a huge piece of land will splinter into a variety of sub-groups which diverge from one another in worldview. Hence, it cannot be seen a priori that a thorough ethical construction not afflicted by insurmountable tensions can't arise within a huge country, but it will be realized when we view through an empirical lens.
Furthermore, I think you're focusing too much on sheer size alone, but you should also take diversity into account. Islamic Arabs, Chinese, Congolese, etc. have very different ideas about what is important in society. Over years, they might assimilate, but more traditional ones will immigrate and take their place. There are two possible solutions, from a moral perspective: 1) stop allowing immigrants and homogenize society; 2) become a true multiculturalist society, as opposed to an assimilationist one. I.e., become a loose confederation of groups that each practices morality as it sees fit. Either one of these has advantages over what we have now, since we are lacking in any semblance of cohesion. All we how now are vacuous platitudes that are hammered into our heads about freedom, equality, and other such buzzwords. These are all but meaningless, falling well short of a morality that a person can live by.
I think of morality like the reflection of a people in a mirror. If they are bound by shared moral judgments, the reflection is something intelligible, but if they are not, it is a jumbled mess.
More people + more resources = more power: the formula for civilisations since the first city-states. The best route to this has been conquest and/or assimilation, thus forming a feedback loop until the bureaucratic machinery of a state buckled or was rebuilt to take the strain. Today, bureaucracy has a near-limitless potential to coerce its subjects. The world's most powerful social and political force - the US federal government - is highly polyglot in fact, and totally polyglot in ideal. My argument is that human groupings will relentlessly tend towards the larger and more abstract, and hence less morally meaningful, because that is where greater power lay.
This argument is acceptable to me. I agree that it is in the interest of the US government to eradicate difference, yet open its arms to everyone and anyone in the hopes of converting him. Our society's goal is to eliminate its citizens' unique features because this will render them without ties to anything but the state, but at the same time, the gov't keeps bringing in people with strong ties to other powers(such as their ethno-nations) because it needs to keep expanding its workforce and consumer base. Abroad, our goal is to crush anyone who impedes our unlimited expansion in any way. This is a point made by Carl Schmitt years ago; namely, liberalism seeks to smooth the political sphere over, so that no one gets in the way of economic growth. It is internationalist, that is to say, and the 'neocons' are staunch liberals. We think of morality as one of the most distinctly human aspects of our kind, so it is only natural that our anti-human government places no value on it and acts in a way that drains it of all power.