Are there any fascist thinkers who oppose militarism? What happens in a fascist state that can't declare war on other states?
'Fascism' has largely degenerated into a term of abuse but if we look at the meaning of 'fascism' as it characterised the original fascist movements - Italy from the 1920s, Portugal and Spain from the 1930s - its four main features were (1) the totalitarian state, (2) the leadership principle, (3) nationalism, and (4) opposition to communism. Separately or in combination these features do not entail militarism in the sense of a predisposition to use military force against other states.
Fascism produced few theorists. Undoubtedly one theorist was Giovanni Gentile (1875-1944) : https://en.wikipedia.org/wiki/Giovanni_Gentile#Gentile's_definition_of_and_vision_for_Fascism.
In Italy, besides Gentile, Gabriele D'Annunzio (1863-1938) exerted a degree of influence on fascist ideology.
The American, Lawrence Dennis (1893-1977), promoted fascist theory in The Coming American Fascism (1936) and The Dynamics of War and Revolution (1940).
On the whole, however, we have to extract fascist theory from fascist practice. Fascism has nothing to match the vast array of theorists that communism, socialism, and capitalism have produced.
There has never been a perfectly totalitarian state but one aim of fascism was to control every aspect of its citizens' lives. Civil society - the existence of intermediate institutions between the citizen and the state - were discouraged or suppressed as far as possible. The church was often too powerful fully to control (and did not need control when it co-operated actively with the regime in Spain and Portugal) but trade unions and voluntary associations of all kinds had no independent legitimacy. From education to birth-rates, from the right to marry to the right to associate, from books and the press to freedom of speech, all were subject to state policy. Nothing that conflicted with state policy was tolerated. Propaganda and violence were equal instruments of the fascist totalitarian state.
The leadership principle
The totalitarian state was a dictatorship. Parliaments, laws, elections, rival political parties, were either manipulated or abolished. Power and authority resided at the centre in one person - in Italy, for instance, in Musssolini; in Spain, Franco. (Mussolini never managed totally to sideline the monarchy, which in part led to his downfall.)
The fascist state was predominantly nationalist rather than strictly racist. This is one reason that many scholars (including the principal scholar of fascism, Roger Griffin) distinguish between fascism and Nazism. ('Nation' is a political and cultural concept; 'race' is pseudo-genetic.) This isn't to say that there were not racist elements in the Italian, Spanish or Portuguese regimes but there were, for instance, Jewish fascists in Italy. Mussolini wanted to promote the glory of the Italian nation and to connect it with its Imperial Roman past. As long as you were loyal to the fascist regime, Mussolini was not primarily concerned with your 'race'. It is true that in the late 1930s under pressure from Nazi Germany, anti-Jewish laws were passed in Italy - disgraceful enough - but they were not enforced with German rigour.
Fascism and anti-Communism
The contrasts and antagonisms were plain. Fascism saw nothing beyond the nation state; Communism was internationalist at least in theory and saw nothing ultimate about the nation state. Communism stressed class struggle; fascism at least projected itself as defending and promoting the single, undivided nation held together by authority and with no inherent conflicts of interest between social classes - or none that mattered compared with national unity.
Fascism and militarism
There is no inherent, conceptual link between fascism, as characterised above, and militarism. Mussolini's regime was militarist as its invasions of Ethiopia, Eritrea, Albania and Greece clearly evidenced. But Spain did not invade other countries, and skilfully kept out of the Second World War. Portugal under Salazar maintained its Empire but did not seek to expand its territory by military or other means.
Probabilistically there is a connexion between fascist nationalism and militarism. It is a fatally easy step from defending the nation state to attacking other states in order to enhance the power or prestige of your own. But this is pragmatics, not inherent logic.
As to your second question, 'What happens in a fascist state that can't declare war on other states?', I'd say that if fascism were inherently militaristic, then an inability to engage in (aggressive) war would defeat one part of fascism's raison d'être. I have argued, however, that militarism is not inherent to fascism.
None of this is to deny that all fascist states employed a domestic policy of coercion, intimidation, violence, murder, terror and suppression. But that is not the same thing as militarism.
O'Sullivan, Noel, Fascism, ISBN 10: 046011428X / ISBN 13: 9780460114288 Published by Everyman Ltd, 1983.
Griffin, Roger, The Nature of Fascism, ISBN 10: 0415096618 / ISBN 13: 9780415096614 Published by Routledge, 1993.
Griffin, Roger, Fascism (Oxford Readers), ISBN 10: 0192892495 / ISBN 13: 9780192892492 Published by Oxford University Press, 1995.
Griffin, Roger, International Fascism: Theories, Causes and the New Consensus, ISBN 10: 0340706139 / ISBN 13: 9780340706138 Published by A&C Black 3PL 1998-02, 1998.
Macdonald, Hamish, Mussolini and Italian Fascism, ISBN 10: 0748733868 / ISBN 13: 9780748733866 Published by Nelson Thornes Ltd, 1999.