**3. Relevant Literature**

Even though there is an extensive body of literature in the area of defense economics addressing the economic effects of military expenditure, studies that examine the effects of military expenditure on environmental sustainability are relatively scarce, despite recently growing interest. In this context, we present the findings of some prominent empirical studies that focus on the nexus of militarization and environmental sustainability. However, the findings of these empirical studies vary due their selection of methodology, sample period, and units. Accordingly, the bulk of these studies were inspired by earlier pioneering attempts [17,27–29,34,35] that argued for the impact of militarization on environmental and sociological grounds.

Earlier empirical efforts aimed to reveal the impacts of militarization on environmental degradation within the context of political economy by accounting for sociological aspects as well. In this context, Jorgenson [17], Jorgenson and Clark [34], and Jorgenson et al. [35] argued for the role of militarization in expanding the ecological footprints of nations within the context of the treadmill of production and the treadmill of destruction theories. Jorgenson [17] analyzed the effects of international power on the ecological footprint of nations by dividing international power into economic power, coerciveness, and dependence on exports. The results of standard cross-sectional regressions revealed that military technological power and dependence on exports have detrimental effects on a nation's ecological footprint, whereas capital intensity has both direct and indirect effects driven by military power, urban population, domestic income inequality, and secondary education. Jorgenson and Clark [34] analyzed the effect of militarization on ecological footprint by incorporating the panel data set of 53 developed and less-developed countries over the period 1975–2000. Utilizing a random-effects estimator, the results indicate that the per capita ecological footprint of a nation is highly dependent on per capita GDP and military expenditure per soldier. Hence, these findings firmly confirm the treadmill of production and treadmill of destruction theories. Likewise, Jorgenson et al. [35] examined whether the treadmill of destruction theory was valid for 72 countries between 1970 and 2000. The conventional

panel data estimates (fixed effects estimator) suggest that militarization has directly positive effects on environmental degradation. By incorporating an annual panel dataset covering 68 countries over the period 1970–2000, Clark et al. [36] validated the treadmill of destruction theory and detected a positive interplay between defense expenditures and energy consumption. They also identified the link between militarization, energy use, and environmental damage from the fuel consumption of military elements. In line with our arguments in this present study, Reis et al. [37] adopted a different method, touching on the relationship between militarization and carbon footprint. Based on systematic literature review and case study researches, the authors find that European Union defense industry plays a crucial role on circular economy.

Some studies examine this relationship in terms of individual countries by employing various time-series methods. To this end, in a relatively earlier attempt, Reuveny et al. [38] drew attention to the destructive effects of militarization and energy use in the USA between 1980 and 2000. Their findings tended to support the treadmill of destruction theory, so that deforestation was associated with increases in conflicts and mass production. For the USA, Bildirici [39] analyzed the effects of militarization on carbon dioxide emissions by employing a bounds test approach to cointegration over the period 1960–2013. The findings showed that, together with economic growth and disposal of energy, militarization has an intensifying effect on CO2 emissions. In addition, the findings of causality tests verified the presence of unidirectional causality from CO2 emissions to militarization, as well as bidirectional causality. By employing the same methodologies, Bildirici [40] derived similar findings for the USA economy between 1984 and 2015. Solarin et al. [41] discussed the role of energy consumption and military expenditures on CO2 emissions for the USA and highlighted the crucial role of militarization in environmental degradation. Ahmed et al. [42] uncovered a long-term interplay between energy consumption, military expenditure, and environmental degradation in Pakistan between 1971 and 2016 by employing novel cointegration tests and bootstrap causality tests. In addition to militarization, Gokmenoglu et al. [43] also investigated the role of financial development on environmental degradation and ecological footprint in Turkey, which is among the leading military spenders in NATO. Incorporating annual data spanning from 1960 to 2014, they employed the FMOLS estimator to find the magnitude and direction of the long-term coefficients. Overall, the findings support the treadmill of destruction theory for Turkey.

Ullah et al. [15] investigated the asymmetric effects of militarization on economic development and environmental degradation for Pakistan and India, which are competing in arms and industrialization, and have extensive disputes over the Kashmir issue. By employing the NARDL model for the period 1985–2018, the authors uncovered the presence of an asymmetric relationship between militarization and environmental degradation for both countries. In this case, a 1% decrease in militarization tended to alleviate carbon emissions by 0.225% in Pakistan, whereas carbon emissions tended to diminish by 0.337% in India for the same decrease. Using a novel empirical approach, Wang et al. [44] scrutinized the influence of militarization on crude oil dependency and CO2 emissions for 11 crude oil net importing countries between 1990 and 2019. The results of Fourier ARDL (FARDL) methodology underlined that, as the largest crude oil consumers, only China and India exhibit a cointegration relationship between crude oil dependence, CO2 emissions, and militarization. Strikingly, among the developed countries, only Italy displayed a cointegration relationship between dependency on crude oil, CO2 emissions, and militarization due to its geography, energy shortage, and high crude oil dependency. For the rest of the sample, Wang et al. [44] found no cointegration relationships. Finally, in a recent paper, Erdogan et al. [45] examined the effect of militarization on environmental sustainability for Mediterranean countries over the period 1965–2019. By conducting global vector autoregression (VAR) analysis, the authors confirmed the validity of the treadmill of destruction theory. Accordingly, an unanticipated positive change in global military expenditure tends to increase carbon emissions.

It should also be noted that there has been recent growing interest among scholars in conducting empirical analyses due to recent advances in panel data analysis. In this context, Ben Afia and Harbi [46] investigated the impacts of militarization on air pollution for 121 countries, covering the period between 1980 and 2011. By conducting instrumental variables analysis, the authors found that militarization had positive direct and indirect effects on per capita emissions for all countries. Bildirici [47] analyzed the interconnectedness of CO2 emissions and militarization for the G7 countries by incorporating annual panel data over the period 1985–2015. In order to reveal the long-term relationship between CO2 emissions, militarization, GDP per capita, and energy consumption, panel autoregressive distributed lag (ARDL) methodology was utilized. The findings demonstrate the long-term interplay between the aforementioned variables.

Furthermore, the findings of panel causality tests confirmed the existence of unidirectional causalities running from militarization and energy consumption to CO2 emissions. Bradford and Stoner [48] aimed to determine the energy use and environmental effects of militarization by using CO2 emission levels, per capita national income, city population, and military expenditures in a panel data analysis for 62 countries between 1975 and 2014, and 162 countries between 1960 and 2014. Working with a fairly large and comprehensive dataset, the authors found that countries with high military expenditures also have high carbon emissions, and that militarization has harmful environmental consequences. In a similar vein, Ben Afia and Harbi [49] examined the direct and indirect effects of militarization through the income channel for 120 countries, spanning from 1980 to 2015. Their findings suggest that military outlays tend to have positive direct and indirect effects on per capita emissions overall. Using a generalized method of moments (GMM) and structural VAR (SVAR) model, Domguia and Poumie [50] scrutinized the relationship between CO2 and methane gases and defense expenditures for 54 African countries between 1980 and 2016. The findings exhibited the presence of positive interplay between environmental damage, militarization, and energy use. Zandi et al. [51] conducted fully modified ordinary least squares (FMOLS) and dynamic ordinary least squares (DOLS) analysis for six Asian countries between 1995 and 2017, uncovering a positive and strong relationship between military expenditures, energy use, and CO2 emissions. Finally, for South Asian countries between 1984 and 2019, Qayyum et al. [52] demonstrated the devastating effects of defense expenditures and energy use on the environment by using panel ARDL and causality tests. Dudzeviˇcut¯ e et al. [ ˙ 53] employed Spearman correlation as well as ALM for Baltic countries, and found a positive relationship between military expenditure and energy use. However, all the studies mentioned in this section excluded the asymmetric relationships when evaluating direct effects. The development of nonlinear methods has provided a remarkable opportunity to reveal networks of relations, such as the damage to the environment from sudden military expenditures, or environmental sustainability during arms races. Therefore, this study aims to make a contribution to this field using the NARDL method.

#### **4. Data Issues, Model Structure, and Estimation Strategy**
